ABSTRACT

VARIABILITY OF HOT SUBDWARF STARS FROM THE PALOMAR-GREEN CATALOG OF ULTRAVIOLET EXCESS STELLAR OBJECTS

This thesis presents a survey for variability in 985 objects identified as hot subdwarf stars by the Palomar-Green Catalog of Ultraviolet Excess Stellar Objects. Catalina Real-Time Transient Survey (CRTS) light curves are used to find the variability. Landolt standard stars that are also hot subdwarf stars in the Palomar-Green Catalog are used to calibrate CRTS photometry in the visual (V) band, and show it is accurate to within ΔV = 0.1 magnitudes between V = 12.0 and V = 16.0. Eleven objects are found to have variability that exceeds four standard deviations above the means of their CRTS light curves. Two are objects already known to be variable. They are PG 1101+385 (Mrk 421), a BL Lac object, and PG 1419+081, a short-period, pulsating sdB star. The other nine are new discoveries of variability: PG 0901+309, PG 0923+329, PG 0934+554, PG 1201+258, PG 1025+244, PG 1411+219, PG 1640+645, PG 1700+315, and PG 2217+059. PG 0934+554 is a spectrophotometric standard of Massey et al. (1988): this discovery of its variability shows that it should not be used as a standard. PG 0934+554 is identified as a BL Lac object. More tentative identifications include PG 1411+219 as an AM CVn star, PG 1640+645 and PG 2217+059 as dwarf novae, and PG 1700+315 as an eclipsing binary star system. Detailed follow-up observations are needed for the other variables, although they may be pulsating hot subdwarf stars.

Melissa Anne Blacketer December 2015

VARIABILITY OF HOT SUBDWARF STARS FROM THE PALOMAR-GREEN CATALOG OF ULTRAVIOLET EXCESS STELLAR OBJECTS

by Melissa Anne Blacketer

A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Physics in the College of Science and Mathematics California State University, Fresno December 2015 APPROVED For the Department of Physics:

We, the undersigned, certify that the thesis of the following student meets the required standards of scholarship, format, and style of the university and the student's graduate degree program for the awarding of the master's degree.

Melissa Anne Blacketer Thesis Author

Frederick A. Ringwald (Chair) Physics

Gerardo Munoz Physics

Douglas Singleton Physics

For the University Graduate Committee:

Dean, Division of Graduate Studies AUTHORIZATION FOR REPRODUCTION OF MASTER’S THESIS

X I grant permission for the reproduction of this thesis in part or in its entirety without further authorization from me, on the condition that the person or agency requesting reproduction absorbs the cost and provides proper acknowledgment of authorship.

Permission to reproduce this thesis in part or in its entirety must be obtained from me.

Signature of thesis author: ACKNOWLEDGMENTS I would like to thank the Department of Physics faculty at California State University, Fresno for their endless support and encouragement I received since day one. I thank the College of Science and Mathematics of California State University, Fresno for a Faculty Sponsored Student Research Award, which allowed me to share my research with the top scientists in the field of astronomy at the American Astronomical Society meeting in Boston. Also much thanks to the Department of Physics of California State University, Fresno for support through a teaching assistantship. Many thanks to the Catalina Real-Time Transient Survey (CRTS) for their photometry used in this research. The Catalina Sky Survey (CSS) is funded by the National Aeronautics and Space Administration under Grant No. NNG05GF22G issued through the Science Mission Directorate Near-Earth Objects Observations Program. The CRTS survey is supported by the U. S. National Science Foundation under grants AST-0909182 and AST-1313422. This research has made use of data obtained from, and software provided by, the US Virtual Astronomical Observatory, which is sponsored by the National Science Foundation and the National Aeronautics and Space Administration (NASA). This research has made use of the SIMBAD database, operated by CDS, Strasbourg, France. This research has also made use of NASA's Astrophysics Data System Bibliographic Services. Last but certainly not least, I would like to thank my family for their endless love and support. To Danny Guice, thank you for believing in me even when I didn’t. I could not have done this without you. TABLE OF CONTENTS Page

LIST OF TABLES ...... vii LIST OF FIGURES ...... viii INTRODUCTION ...... 1 What Are Hot Subdwarf Stars ...... 1 Formation Theories ...... 6 Hot Subdwarf Research ...... 10 RESEARCH PROGRAM ...... 14 Palomar-Green Catalog ...... 14 Catalina Real-Time Transient Survey ...... 16 Types of Variability ...... 17 Method ...... 18 OBSERVATIONS AND ANALYSIS ...... 20

Analysis Part I ...... 20 Analysis Part II ...... 22 Calibration ...... 23 List of Standard Stars ...... 26 List of PG Hot Subdwarf Stars Part I ...... 29 List of PG Hot Subdwarf Stars Part II ...... 49 RESULTS AND DISCUSSION ...... 73 PG 0934+554 ...... 73 PG 1101+385 ...... 75

PG 1025+244 ...... 75 PG 0923+329 ...... 76 vi vi Page

PG 1201+258 ...... 76

PG 0901+309 ...... 78 PG 1640+645 ...... 78 PG 1419+081 ...... 79 PG 2217+059 ...... 80 PG 1700+315 ...... 81 PG 1411+219 ...... 82 CONCLUSION ...... 84 REFERENCES ...... 87

LIST OF TABLES

Page

Table 1: PG highest stars on Figure 5...... 21 Table 2: PG hot subdwarf stars and related objects significant variance ...... 21 Table 3: PG highest variable stars on Figure 6...... 23 Table 4: PG hot subdwarf stars and related objects significant variance ...... 23 Table 5: PG Standard Stars from Figure 7...... 26 Table 6: PG Standard Stars from Figure 5...... 29 Table 7: PG Standard Stars from Figure 6...... 49 Table 8: The candidates ...... 86

LIST OF FIGURES

Page

Figure 1. Diagram of the stellar evolution of a typical 1 solar mass star. The evolution starts on the MS curve, and progresses to the red giant branch (RGB), to the horizontal branch, to the AGB, planetary nebula phase, and down to the last stage white dwarf stars. (Carrol, nd B., Ostlie, D., 2007, Introduction to Modern Astrophysics, 2 ed.) ...... 4 Figure 2. This Hertzsprung-Russell diagram shows the placement of the hot subdwarf stars sdB and sdO on the extreme horizontal branch (EHB). (Heber U. 2009, ARAA, 47, 211) ...... 5 Figure 3. The theoretical formation channels for hot subdwarf stars: Roche lobe overflow, Common Envelope phase, and two white dwarf mergers. (Heber 2009) ...... 7

Figure 4. Roche lobe equipotential surface diagram of two masses M1 and M2. L1, the inner Lagrangian point is pictured between the two masses, at the apex of the tear-drop shape. The arrows indicate direction of force a test mass would experience at that location. (Boehm-Vitense 1989) ...... 8 Figure 5. This figure shows the first half of each star's average magnitude against their standard deviation...... 20 Figure 6: This figure shows the second half of each star's average magnitude against their standard deviation ...... 22 Figure 7. Average magnitude of the PG standard stars from Landolt (1992a, 1992b) plotted against each stars standard deviation ...... 25

Figure 8. Light curve of PG 0934+554...... 74

Figure 9. Light curve of PG 1101+385...... 75

Figure 10. Light curve of PG 1025+244...... 76

Figure 11. Light curve of PG 0923+329...... 77

Figure 12. Light curve of PG 1201+258...... 77

Figure 13. Light curve of PG 0901+309...... 78

Figure 14. Light curve of PG 1640+645...... 79

Figure 15. Light curve of PG 1419+081...... 80 ix ix Page

Figure 16. Light curve of PG 2217+059...... 81

Figure 17. Light curve of PG 1700+315...... 82

Figure 18. Light curve of PG 1411+219...... 83

INTRODUCTION

Stars are formed within clouds of gas and dust that collapsed upon themselves due to their own gravity. As a cloud condenses its center starts to heat up and if there is sufficient mass the core will eventually become hot enough to ignite nuclear fusion. The differences between stars, for example, mass, density, temperature, or surface gravity, can lead to extraordinarily different lives. This is especially evident in the variety of ways in which stars live out the end of their lives and eventually, their deaths. A star that is born with low mass and a star that is born with high mass will have dramatically different outcomes. The late stages of stellar evolution are critical to understanding the stellar evolution of stars. It gives insight into how the stars are formed, as well as the future state of similar stars and the evolution of galaxies.

What Are Hot Subdwarf Stars The Hertzsprung-Russell diagram, or H-R diagram, was created in 1912 by Ejnar Hertzsprung, and refined in 1913 by Henry Norris Russell. It is a plot of luminosity, or power output, as a function of temperature for stars. Since absolute magnitude measures luminosity, and since spectral type is determined by temperature, the H-R diagram gives astronomers insight into stellar evolution. The H-R diagram can be plotted with surface temperature (with higher temperatures towards the left), spectral classification (OBAFGKM where O-types are the hottest and M-types are the coolest stars), or color on the horizontal axis, versus absolute magnitude or luminosity (in comparison to the Sun) on the vertical axis. Other information the H-R diagram provides includes stellar mass and radius. There are five main regions where stars group together. The diagonal grouping that runs from the upper left to the lower right is called the main 2 2 sequence. Main sequence (MS) stars are all stars that have nuclear fusion in their cores, in which hydrogen is being converted into helium. Most stars spend a majority of their lives on the main sequence, and depending on their initial masses can finish their lives quite differently. The mass of the stars, although not explicitly on the H-R diagram, are still inferred by a star’s position on the main sequence curve. One can calculate a MS star’s mass from the luminosity, called the mass-luminosity relation. This mass-luminosity relationship for MS stars is mathematically expressed as:

퐿 푀 = ( )3.5 퐿푆푢푛 푀푆푢푛

From the expression, the bigger the star’s mass, the brighter the star, and the masses increase to the top left of the curve. While a star is on the MS, it is maintaining hydrostatic equilibrium where the hot core produces thermal pressure pushing outwards and is equally balanced by the inward pull from the gravitational collapse of the gas. Just above the MS curve is the region of stars called Giants. These stars have larger radii, as the name suggests, and in general greater luminosity than MS stars. The more massive stars will live much shorter lives, as they will burn through energy much quicker (since E = mc2). A star will become a giant star after leaving the MS. To leave the MS means the star has started to lose hydrostatic equilibrium. This is because stars only have a limited supply of hydrogen for fusion, and at that transition point, has depleted its hydrogen supply in the core. Now there will be more helium in the core, and the star will begin fusion of the shell of hydrogen surrounding the core. The core will begin to contract and the outer layers will expand. 3 3

A star can also leave the MS and join the Supergiant branch, which is located above the Giant branch. Where a star ends up directly after the MS curve depends on the stars’ initial mass. After becoming a giant star or supergiant star, the star has several evolutionary channels. A star like the Sun will eventually become a white dwarf, where its outer layers are thrown outward as a planetary nebula, leaving behind the dense core. Stars that are ten times (or more) the mass of the Sun will explode in a supernova, leaving behind a neutron star (a star made entirely of neutrons), or a black hole. White dwarf stars are seen below the MS curve. As mentioned above, a white dwarf is the stellar remnant, the core of a star, which has expelled its outer layers. It is composed of electron-degenerate matter, which is where electrons are so densely packed together that they stop the collapsing of matter because of the Pauli Exclusion Principle. For a diagram representation of the stellar evolution of a 1 solar mass star, see Figure 1. Most of these diagrams do not however, depict the star that is the topic of this thesis: hot subdwarf stars. Since the discovery of hot subdwarf stars, the Hertzsprung-Russell diagram has been modified to include these stars. The modified diagram is pictured in Figure 2. Hot subdwarf stars are found at the blue end of the extreme horizontal- branch (EHB) which lies between the main sequence and the white dwarf sequence. It is believed the EHB stars are the evolutionary step after a star has undergone the red giant phase. There are two main types of these hot subluminous stars. The spectral type O subluminous stars (sdO) and the spectral type B subluminous stars (sdB). These stars have canonical masses of ~0.5 M☉, blue colors, effective temperature (푇푒푓푓) from 20,000 K to 100,000 K, and logarithm of surface gravity (log g) between 4 4

Figure 1. Diagram of the stellar evolution of a typical 1 solar mass star. The evolution starts on the MS curve, and progresses to the red giant branch (RGB), to the horizontal branch, to the AGB, planetary nebula phase, and down to the last stage white dwarf stars. (Carrol, B., Ostlie, D., 2007, Introduction to Modern Astrophysics, 2nd ed.)

5 5

Figure 2. This Hertzsprung-Russell diagram shows the placement of the hot subdwarf stars sdB and sdO on the extreme horizontal branch (EHB). (Heber U. 2009, ARAA, 47, 211)

4.0 to 6.5 dex. In general, therefore, hot subdwarf stars have higher surface gravity than the Sun, with log g = 4.438 dex. They are core helium-burning stars with a thin hydrogen envelope, and are thought to evolve directly into white dwarf stars by avoiding the asymptotic giant branch (AGB) (Heber et al. 1984; Heber 1986). In other words, as these stars die, they increasingly resemble hot rocks, and become less like gaseous MS stars, such as the Sun. SdB stars spectroscopically form a homogenous class, have hydrogen dominated atmospheres, 푇푒푓푓 between 20,000 K and 40,000 K, log g between 5.2 to 6.5 dex. SdO stars have a wide 6 6 variety of spectra, have helium rich atmospheres, 푇푒푓푓 between 40,000 K and 100,000 K, and a log g between 4.0 and 6.5 dex (Oreiro et al. 2004). It is unknown how these stars were able to lose enough mass to leave behind such a thin inert hydrogen layer after leaving the red giant branch. During the last stages in a red giant star, the hydrogen burning shell surrounding the core will continue to add helium into the core. As it continues to dump helium into the core, the core will continue to heat up. Eventually, the collapsing core will be so densely packed, that the core will become degenerate, halting the contraction. Yet the hydrogen burning continues, dropping more helium into the core, continuing to heat up the core, until there is a thermonuclear runaway. This is the helium flash. The helium is ignited and burned for just a few minutes but can release as much energy as an entire galaxy. The helium flash makes the star no longer degenerate, and then the star behaves as a gas again (Schneider and Arny 2009). After this, the red giant star moves to the horizontal branch, which is not the EHB but lies (not pictured) roughly between the giants and MS curve. It is a horizontal branch of stars with roughly the same luminosities but different temperatures. After this, the star would then move up to the AGB. It is thought the hot subdwarf phase occurs right after the red giant phase, but in this case, the star has skipped the AGB phase and evolves into a white dwarf. Therefore, the red giant star must have lost all of its envelope at the tip of the red giant branch to make a hot subdwarf star.

Formation Theories Currently, the exact channels that lead to the formation of hot subdwarf stars are still unknown. There are, however, three main leading theories: Roche Lobe overflow, Common Envelope phase, and two white dwarf mergers. The 7 7 formation channels of hot subdwarf stars continue to be extensively studied. Refer to Figure 3 for a graphical representation of the theoretical formation channels.

Figure 3. The theoretical formation channels for hot subdwarf stars: Roche lobe overflow, Common Envelope phase, and two white dwarf mergers. (Heber 2009)

Most stars in the sky are in binary systems, which means two or more stars orbit around a common center of mass. Binary interaction between stars could result in the formation of hot subdwarf stars since it could explain the required mass loss. Binaries can be identified in several ways: resolving them visually, spectroscopically, photometrically, or in eclipsing systems. Since most stars are found to be in pairs or multiples, it is critical to understand how these stars interact with each other. If the distance between the two stars in a binary system is not too 8 8 great, one phenomenon that could account for how hot subdwarf stars are formed is called Roche Lobe overflow (RLOF). There is a region of space around a star where gas will be gravitationally bound to that star. Consider the situation where there are two stars orbiting one another about their common center of mass, a relatively short distance apart compared to their sizes, and orbit in a circular path. The reference frame will be rotating with the binaries with the coordinate system centered at the common center of mass. Close to each star’s center of mass, the gravitational equipotential is spherical. Further out, the equipotential surface will distort, as gravity from the stars begin to affect one another (Paczynski 1971). The distortion caused by the two stars will be roughly teardrop shaped and are referred to as Roche lobes. Figure 4 shows the equipotential surfaces of this scenario.

Figure 4. Roche lobe equipotential surface diagram of two masses M1 and M2. L1, the inner Lagrangian point is pictured between the two masses, at the apex of the tear-drop shape. The arrows indicate direction of force a test mass would experience at that location. (Boehm-Vitense 1989) 9 9

The shape of the Roche lobe depends on the mass ratio. Paczynski noted that one can measure the size of the Roche lobe by “the average radius r which is defined so that 4 휋푟3 is equal to the volume within the Roche lobe” (Paczynski 3

1971). The formulae by Paczynski for the radius are calculated as follows:

푟 푀 푀 1 = 0.38 + 0.2푙표푔 1 for 0.3 < 1 < 20 퐴 푀2 푀2

1 푟1 2 푀1 푀1 = ( )3 for 0 < < 0.8 퐴 4 푀 +푀 푀 33 1 2 2 The point where the two potential surfaces touch between the two stars, is called the inner Lagrangian point L1. At this point, there is no net force acting on any mass at this point. The mass will stay at that position relative to the two stars and rotate with them. This location is unstable, however, and a slight perturbation will cause the mass to move.

If the star fills its Roche lobe up to the L1 Lagrangian point, then the companion star can then accept the over flowing mass and accrete this mass. This dynamical mass transfer can account for the thin hydrogen envelope observed in hot subdwarf stars (Han et al 2002, 2003). It is possible that both stars can fill or overflow their Roche lobes. If one star is transferring matter to a companion star by means of RLOF, the companion star may not be able to accrete all the gas. The gas will then begin to build up and eventually fill its own Roche lobe. This is called the Common Envelope phase. There are now two (or more) stars that share an envelope of gas. Since the binary system is sharing this envelope, inside there is friction as the two stars orbit each other inside the matter. The stars will begin to spiral into each other and eventually this complex process will give orbital energy to the envelope and eject it 10 10

(Paczynski 1976). The ejection of the common envelope can account for the mass loss. The other theoretical possibility is in the merger of two helium white dwarf stars. The two stars will merge together to produce a single star, theoretically the hot subdwarf star. The formation of these single hot subdwarf stars were carefully calculated and modeled by Han et al (2002, 2003). It is theorized that should two helium white dwarf stars converge, and the merging be hot enough, it could ignite helium causing a helium flash. The result after which being a single sdB star (Saio and Jeffery 2000).

Hot Subdwarf Research There are many reasons to study hot subdwarf stars. Hot subdwarf stars are found in the disk and bulge of the galaxy, in elliptical galaxies, and in globular clusters. Hot subdwarf stars explain the UV-upturn, show insight into stellar interiors by studying their pulsations, and dominate the sky for magnitudes brighter than B = 18 in surveys of faint blue objects. A brief review follows. Hot subdwarf stars are found in globular clusters. In NGC 6752, these spectra were identical to the spectra of the hot subdwarf stars in the field of our galaxy (Heber et al. 1986). Globular clusters are very old, thought to have condensed soon after the Big Bang itself, which implies that hot subdwarf stars can also be very old. They can also be found in elliptical galaxies (Brown et al. 2008) and in the galactic bulge (Zoccali et al. 2003), which are both also old stellar populations. An unexpected discovery in early-type galaxies is called the UV-upturn. It is an excess of ultraviolet light coming from these galaxies. This is unexpected because elliptical galaxies have little to no star formation, so they aren’t expected 11 11 to have many hot stars. The mystery was solved when observations concluded the UV-upturn was due to the prominence of hot subdwarf stars (Brown et al. 1997). More perplexing are the cases in which gas-giant planets orbit a hot subdwarf. This is astonishing, since to do this, the gas-giant planet must have undergone the red giant phase and survived. An example of this is a giant planet that orbits the star V391 Pegasi, a pulsating subdwarf B star (Silvotti et al. 2007). Studying pulsations in stars allows investigation into their interiors, in much the way seismology shows geologists what is inside Earth. This is called asteroseismology. Some hot subdwarf stars pulsate, and so are of interest to asteroseismology. The first of the two main types of hot subdwarf stars that were discovered to pulsate were the sdB stars (Kilkenney et al. 1997). These pulsations have low amplitudes and are multi-periodic, reaching around a few mmag (milli- magnitudes), with short periods between 80 and 600 seconds. Interestingly, these pulsations were theorized at about the same time (Charpinet et al. 1996). These stars are now known as V361 Hya stars (Kilkenney 2007). The discovery of long- period pulsations has since followed from the discovery of the short-period pulsators. The long-period pulsating sdB stars have periods ranging from 2000 to 9000 seconds. These stars are now known as V1093 Her stars (Green et al 2003). SdO stars have also been observed to pulsate. The short-period pulsations are due to p-mode (pressure mode) pulsations. P-mode pulsations are due to the 휅 mechanism, which is when the opacity of a layer in the star increases with compression. Somewhere in the star, some of the stellar material gets pulled in by gravity and falls inward. As the material falls inward, the material compresses and the temperature increases. The heated-up material becomes opaque to radiation, trapping the heat. The temperature will continue to increase until there is enough pressure to push back outwards. Since 12 12 this is a gas, it will expand, cool, and become more transparent, releasing the energy and the pressure that was below. Gravity wins again and pulls down the material and the cycle repeats. Hence, pulsation. However, the Kramers Law 휌 휅 ∝ 푇3.5 shows that as the material moves inward, increasing the density and temperature, the opacity 휅 will become smaller. This means the opacity actually decreases with compression. Yet, we do see such pulsations in stars, so the opacity must increase with compression, perhaps under special conditions (Carrol and Ostlie, 2007).

S.A. Zhevakin first found the conditions necessary for the 휅 mechanism to work as is needed. There are partial ionization zones where some of the work done on the gases is spent on ionizing gases rather than increasing the temperature. Density in Kramer’s Law now dominates and the mechanism works (Carrol and Ostlie, 2007).

The long-period pulsations are due to g-mode (gravity mode) oscillations. These oscillations are produced by internal gravity waves (Carrol and Ostlie, 2007). The gravity tends to smooth out the material but there are differences in pressure and composition, and gravity will act on those differently. There is an oscillatory motion as the gravity acts as the buoyant force to restore the star to equilibrium. Each type of oscillation provides its own insight into the stellar structure of the star. The p-mode oscillations provide information on the surface of the star while the g-mode oscillations probe deep inside the interior of the star. Thereby studying the pulsations of hot subdwarf stars gives valuable information about the conditions on the inside of the star and the surface. 13 13

Pulsating hot subdwarf stars do not have to confine to one or the other though. There are a small handful of cases of hot subdwarf stars that pulsate in both p and g-modes (Schuh et al. 2006).

RESEARCH PROGRAM

This thesis reports a survey to find variability in the brightness, or apparent magnitude, of a set of hot subdwarf stars. We selected the hot subdwarf stars from the Palomar-Green Catalog of Ultraviolet Excess Stellar Objects, described below. For each hot subdwarf star, we examined whether their apparent magnitudes changed as a function of time. A plot of a star’s apparent magnitude as a function of time is called its light curve. Light curves measured from the Catalina Real- Time Transient Survey (CRTS) (Drake et al. 2009), which is funded by the National Science Foundation and so is publicly available. The CRTS light curves were made in visible light, and are a series of hundreds of apparent magnitudes of the stars, measured on the average of about every 10 days for 8-9 years, beginning in 2005-2007. For each CRTS light curve, the mean apparent magnitude was calculated, as well as the standard deviation of the variance about the mean. This tells how much the star changes in brightness. The next few sections will discuss the types of variability we can expect, the Palomar-Green Catalog, the Catalina Real-Time Transient Survey, and the calibration that shows how accurate the CRTS is for this purpose.

Palomar-Green Catalog The Palomar-Green (PG) Catalog of Ultraviolet Excess Stellar Objects was published by Richard Green, Maarten Schmidt, and James Liebert in 1986 (Green et al. 1986). The survey looked specifically for quasars, although it is dominated by hot, high-gravity stars in the late stages of stellar evolution, such as hot subdwarf stars and white dwarf stars. All these objects are blue in color and have ultraviolet excesses, which means that they have a larger ratio of the intensities of 15 15 ultraviolet-to-blue light than even the hottest normal stars have. The PG survey covered 10,714 square degrees, about 1/4th the celestial sphere, using the Palomar Schmidt 18-inch telescope. More than half the survey, 53%, consisted of hot subdwarf stars, which included sdB and sdO types. The rest consisted of hot white dwarf stars, cataclysmic variables, planetary nebulae, quasars, and other extragalactic objects. A total of 1874 objects of all kinds were found by the Palomar-Green survey, and hence are listed in the Palomar-Green Catalog (Green et al. 1986). Green et al (1986) classified the hot subdwarfs and white dwarfs with the following spectral classifications, some of which they devised themselves: sdB: Subdwarf B stars show high-gravity Balmer series absorption over a wide range of colors. sdB-O: sdB stars with a suggestion of He I 휆4471 in absorption. sd: Usually implies a lower signal-to-noise observation in which two or three Balmer absorption lines of moderate gravity are present. sdOA: Not conventional sdO stars, but showing spectra with dominant hydrogen Balmer absorption along with pronounced He I 휆4471 and often He I 휆4026. sdOB: Spectrum dominated by He I and He II lines and generally showing hydrogen Balmer absorption. sdOD: Cooler subdwarf stars with “pure” He I absorption spectra, characterized by the weakness or absence of hydrogen Blamer lines and He II 휆4686, while showing the singlet 휆4388 about equal in strength to the triplet 휆4471. sdO: A spectrum in which He II 휆4686 and often He I 휆4471 were identified, at a signal to noise ratio somewhat too low for a more detailed description. 16 16

DAn: Have hydrogen Balmer absorption at the very high surface gravities characteristic of hot white dwarf stars. The subtype numbers (n) are the indices from 0 to 9 denoting effective temperature adopted by Sion et al. (1983). DBn: Degenerates with spectra showing only neutral helium absorption. DAO, DAB, DBA: Degenerates with atmospheres mixed with hydrogen and helium composition. O denotes the presence of He II 휆4686, B denotes He I. The order of the letters denotes the dominant atmospheric constituent first. DO: Helium atmosphere degenerates with spectra dominated by ionized helium absorption. DC, DZ: Helium-atmosphere white dwarf stars. These stars show no weak carbon features, they do show metallic features. Note: caution is advised due to incompleteness in this color range, and higher signal-to-noise spectra not provided.

Catalina Real-Time Transient Survey The Catalina Real-Time Transient Survey (CRTS) is an astronomical survey that uses three ground-based telescopes to search for transient objects. Its original purpose was to search for near-Earth asteroids, but since making their data publicly available, many other purposes are being found. It is the precursor for the giant, deep, time-resolved surveys of the future, such as the Large Synoptic Survey Telescope, currently under construction in Chile and among the national observatories’ largest projects. The CRTS uses three ground based telescopes, which are the Catalina Sky Survey (CSS), the Mount Lemmon Survey (MLS), and the Siding Springs Survey 17 17

(SSS). Each telescope has its own range of sky it surveys, and each telescope avoids the Galactic plane due to the crowding of stars. Together, the three telescopes cover about 33,000 square degrees of sky. The CRTS takes photometry from unfiltered images then does a transformation to V, the visual band. This is highly dependent on source color. The color correction for blue objects is small but can be very large for red objects (Drake et al. 2009). Converting the unfiltered apparent magnitudes to V are done with the transformation equations of Bessel and Brett (1988). The comparison stars used are G0-G8 dwarf stars from the all-sky 2MASS Point Source Catalog (Cutri et al. 2003). It is noted that these comparison stars as well as the transformation equations used were chosen for the best choice for asteroids. Caution is advised for using apparent magnitudes from CRTS brighter than 13th magnitude (V ~ 13), due to source saturation and nonlinearity of the response (Drake et al. 2009).

Types of Variability There exists a great variety of variability in stars that is valuable to many subfields in astronomy. A variable star is one that has luminosity or spectroscopic changes. This may be due to the natural evolution of a star’s life, where the star enters into a phase in its life where it pulsates. A star can be variable because it has a companion star that orbits it and the eclipsing dip in their light curves is observed. There is the irradiation effect where a hot star heats up the facing side of a companion. Erupting stars are called cataclysmic variables (CV), where a low- mass, main-sequence star loses mass to a WD companion. Pulsating stars are of central interest to asteroseismology, as described above. 18 18 Method Investigation into finding candidates for variability of the hot subdwarf stars from the PG catalog started with analysis of the data from CRTS. A linear least squares regression was fitted against the data with the goal to minimize the squared error. The least squares regression will be the model used to predict how much a stars’ brightness should vary based on the average magnitude of the star. This is the best fit line for the data.

The equation of the regression curve is 푓(푥, 훽) = 훽0 + 훽1푥. The solution therefore minimizes the squared error: 푘 2 푆퐸 = ∑ 푟푖 푗=0

where r is the residual. The difference between the data and the predicted value by the model is:

푟푖 = 푦푖 − 푓(푥푖, 훽)

Minimizing the squared error is essential since the regression curve should be as close to the data as possible. The closer the line fits the data, the better the model. Therefore, finding a line that will reduce the distance between each data point and the predicted value, will produce the best model. Once the best fit line is computed, then the standard deviation about the mean can be calculated. For residuals, the standard deviation (휎) is computed as follows: 2 ∑(푦푖 − 푓(푥 , 훽)) 휎 = √ 푖 푛 − 2

To make the analysis more easily manageable, the sample of 985 stars was split in approximately half. For each half, a plot was made of the variability of 19 19 each star (see Figures 5 and 6). In both these plots, the variability for each star was found by performing a least-squares linear regression, with standard deviations calculated for the light curves of the individual stars. To find the best candidates for variability, we plotted each star’s standard deviation about the mean of its light curve versus its mean apparent magnitude. The stars with statistically significant variability will have the highest standard deviations. A discussion on how to determine which stars are statistically significant follows in the next section.

OBSERVATIONS AND ANALYSIS

Analysis Part I Figure 5 shows the results of plotting the stars magnitude against the standard deviation. Table 1 lists the labeled stars in order with the PG star identifier. Table 2 lists each star’s standard deviation. The higher the standard deviation the more statistically significant the star’s variability becomes. From the discussion earlier about CRTS, a calibration must be performed in order to ensure the data are accurate.

Figure 5. This figure shows the first half of each star's average magnitude against their standard deviation.

21 21

Table 1: PG highest stars on Figure 5.

Label Identifier A PG 0934+554 B PG 1101+385 C PG 1025+244 D PG 0923+329 E PG 1201+258 F PG 0901+309

Table 2: PG hot subdwarf stars and related objects significant variance

4 휎 5 휎 +6 휎 PG 1025+244 PG 0923+329 PG 0901+309

PG 1101+385 PG 0934+554

PG 1201+258

22 22 Analysis Part II Figure 6 shows the results of plotting the stars magnitude against the standard deviation. Table 3 lists the labeled stars in order with the PG star identifier. Table 4 lists each star’s standard deviation.

Figure 6: This figure shows the second half of each star's average magnitude against their standard deviation

23 23

Table 3: PG highest variable stars on Figure 6. Label Identifier

A PG 1640+645 B PG 1419+081 C PG 2217+059 D PG 1700+ 315 E PG 1411+219

Table 4: PG hot subdwarf stars and related objects significant variance 4 휎 5 휎 +6 휎 PG 1411+219 ------PG 1419+081

------PG 1640+645

PG 1700+315

PG 2217+059

Calibration In order to determine whether a star is variable, one needs to compare their apparent magnitudes to those of stars that do not either intrinsically nor extrinsically change brightness. A standard star in theory will have no variance. However, stars always how some small amount of variability, due to scintillation from differential refraction from turbulence in Earth’s atmosphere, which is also called twinkling. This variation can be accounted for, however. Landolt compiled an extensive and carefully conducted UBVRI photoelectric survey for standard 24 24 stars along the celestial equator (Landolt 1992a, 1992b). The observations produced a list of standard stars between V = 11.5 and V = 16.0. The search for standard stars continued and more standard stars have since been identified and increased the range of magnitudes between V = 8.90 and V = 16.30 (Landolt 2009). Some of Landolt’s standard stars had ultraviolet excesses such that they were included in the PG catalog. In order to calibrate the photometry from the CRTS, only the PG standard stars were chosen for the calibration, as they would be a natural choice to which to compare our list of hot subdwarf stars. Figure 7 is a graph showing this calibration of the photometry of the PG standard stars. We plot the V magnitude as measured by the CRTS against the variance about the mean for each star. A true standard star should have no variance in brightness, hence since there does appear to be some small variance in the brightness of the standard stars in Figure 7, this is attributed to atmospheric extinction and scintillation. Other possible causes for variance are due to unfiltered photometry transformations to V magnitude, and the calibration from G dwarf stars. Magnitudes brighter than V = 13 are to be viewed with caution, as those stars are bright enough to have saturated the detectors used by CRTS, which may explain why there is higher variance on the bright end of Figure 7 (Drake et al. 1988). So long as the hot subdwarf stars magnitudes and their standard deviations are higher than the noise depicted here, the photometry can be trusted for accuracy. Figure 7 shows a total standard deviation of within ΔV = 0.1 magnitudes between V = 12.0 and V = 16.0.

25 25

Figure 7. Average magnitude of the PG standard stars from Landolt (1992a, 1992b) plotted against each stars standard deviation

26 26 List of Standard Stars

Table 5: PG Standard Stars from Figure 7. IDENTIFIER 흈 NOTE

PG 0029+024 15.19 0.022

PG 0039+049 12.83 0.080

PG 0220+132 14.67 0.072

PG 0220+132A 15.41 0.070

PG 0220+132B 13.8 0.095

PG 0231+051 16.07 0.036

PG 0231+051A 12.6 0.092

PG 0231+051B 13.92 0.018

PG 0231+051C 13.5 0.020

PG 0231+051D 13.6 0.018

PG 0231+051E 13.62 0.016

PG 0942-029 13.35 0.019

PG 0942-029B 13.92 0.018

PG 0942-029C 13.41 0.020

PG 0942-029D 11.92 0.127

PG 1034+001 13.25 0.056

PG 1047+003 13.48 0.044 V* UY Sex

PG 1047+003A 13.3 0.042

PG 1047+003B 14.59 0.070

PG 1047+003C 12.24 0.108

PG 1323-086 13.58 0.139

PG 1323-086B 13.23 0.140

PG 1323-086C 13.87 0.128 27 27

Table 5. (cont.) IDENTIFIER 흈 NOTE

PG 1323-086D 11.91 0.245

PG 1407-013 13.77 0.018

PG 1407-013A 14.3 0.018

PG 1407-013B 12.34 0.124

PG 1407-013C 12.36 0.121

PG 1407-013D 14.61 0.017

PG 1407-013E 14.95 0.019

PG 1514+034 13.92 0.020

PG 1525-071 14.69 0.266

PG 1525-071A 13.24 0.111

PG 1525-071B 16.07 0.107

PG 1525-071C 13.17 0.116

PG 1525-071D 16.05 0.217

PG 1633+099 14.35 0.137

PG 1633+099A 14.93 0.033

PG 1633+099B 12.65 0.130

PG 1633+099C 12.89 0.100

PG 1633+099D 13.5 0.040

PG 1633+099E 12.87 0.314

PG 1633+099F 13.43 0.046

PG 1633+099G 13.51 0.041

PG 1647+056 14.64 0.029

PG 1657+078 14.88 0.035 28 28

Table 5. (cont.) IDENTIFIER 흈 NOTE

PG 1657+078A 13.54 0.037

PG 1657+078B 14.38 0.034

PG 1657+078C 14.85 0.035

PG 1657+078E 14.17 0.034

PG 2213-006 14.01 0.023

PG 2213-006A 13.87 0.021

PG 2213-006B 12.37 0.109

PG 2213-006C 14.8 0.020

PG 2213-006E 13.49 0.026

PG 2213-006F 12.3 0.107

PG 2317+046 12.78 0.086

PG 2331+055 15.11 0.128

PG 2331+055A 12.83 0.144

PG 2331+055B 14.42 0.083

PG 2336+004 15.79 0.069

PG 2336+004A 11.11 0.209

PG 2336+004B 12.3 0.105

PG 2349+002 13.29 0.034

29 29 List of PG Hot Subdwarf Stars Part I

Table 6: PG Standard Stars from Figure 5.

IDENTIFIER 휎 CSS MLS SSS NOTE PG 0004+133 0.054 12.99

PG 0005+178 0.202 16.04 16.21

PG 0006+119 0.034 14.54

PG 0007+025 0.202 16.04 16.21

PG 0008+185 0.145 16.6

PG 0009+191 0.039 14.64

PG0009+036 0.105 13.15 13.27

PG 0011+221 0.025 13.41

PG 0011+283 0.069 12.61

PG 0014+068 0.072 15.95 16.07

PG 0016+285 0.037 15.93

PG 0020+240 0.023 15.45

PG 0022+087 0.042 15.52 15.46

PG 0023+298 0.041 15.09

PG 0023+299 0.029 15.6

PG 0024+133 0.055 15.51 15.52

PG 0026+047 0.044 15.02 21.05

PG 0027+222 0.566 20.76 SD

PG0032+247 0.086 14.06

PG 0033+266 0.053 14.3

PG 0038+199 0.026 14.52

PG 0039+049 0.086 12.81 12.93

30 30

Table 6. (cont.)

IDENTIFIER σ CSS MLS SSS NOTE

PG 0039+103/4 0.038 15.31

PG 0039+135/4 0.085 13.57

PG 0042+095 0.041 15.92

PG 0042+211 0.035 15.83

PG 0045+251 0.051 16.39

PG 0046+247 0.086 16.64

PG 0046+207 0.023 14.58

PG 0048+004 0.042 15.58 15.9

PG 0048+091/2 0.039 14.14 14.07

PG 0050+222 0.022 15.34

PG 0050+201 0.221 18.92

PG 0051+169 0.031 15.73

PG 0052+058 0.070 15.99

PG 0053+239 0.030 15.9

PG 0055+016 0.052 15.14 15.25

PG 0057+155 0.108 11.95

PG 0101+040 0.139 12.06

PG 0102+261 0.494 20.63 SD

PG 0105+276 0.016 14.3

PG 0108+195 0.030 14.65

PG 0108+209 NAOF

PG 0110+262 0.068 12.77

PG 0112+142 0.026 15.25 15.26

31 31

Table 6. (cont.)

IDENTIFIER σ CSS MLS SSS NOTE

PG 0113+259 0.031 14.37

PG 0116+242 0.204 18.42

PG 0112+065 0.062 16.43 16.53

PG 0122+214 0.065 12.77

PG 0123+159 0.048 14.26 14.18

PG0132+151 0.022 15.14

PG0133+114 0.117 12.26 12.29

PG 0135+242 0.026 14.94

PG 0142+183 0.028 15.44

PG 0142+148 0.058 13.67 13.8

PG 0154+204 0.020 15.26

PG 0154+182 0.025 15.49

PG 0200+131 0.099 14.53 14.62

PG 0205+13 0.067 14.26 14.48

PG 0206+225 0.017 14.02

PG 0207+005 0.030 15.42 15.49

PG 0208+015 0.035 13.7 13.8

PG 0209-015 0.027 14.05

PG 0209+017 0.025 14.2 14.25

PG 0212+231 0.047 15.6 15.65

PG 0212+148 0.026 14.4 14.51

PG 0212+143 0.507 14.51 14.64 SD

PG 0215+183 0.871 21.74 SD

32 32

Table 6. (cont.)

IDENTIFIER σ CSS MLS SSS NOTE

PG 0216+032 0.024 14.58 14.6

PG 0216+246 0.052 16.34 16.4

PG 0217+241 0.077 16.7 16.8

PG 0217+155 0.044 14.89 14.93

PG 0219+241 0.048 16.23 16.27

PG 0220+132 0.062 14.65 14.62

PG 0226+151 0.092 15.96 15.78

PG 0240+066 0.193 15.97 16.42

PG 0242+132 0.061 13.1 13.21

PG 0248+054 0.097 16.18

PG 0322+078 0.036 15.36

PG 0749+658 0.196 12.09

PG 0752+770 BSL

PG 0806+516 0.033 14.95

PG 0806+682 0.045 15.75

PG 0812+478 0.046 15.17

TYC 3418-1452-1 0.219 12.06

PG 0816+314 0.039 15.62

PG 0821+306 0.058 15.19

PG 0822+645 0.029 15.43

PG 0823+546 BSL

PG 0823+465 0.020 14.53

PG 0823+499 0.133 12.09 11.73

33 33

Table 6. (cont.)

IDENTIFIER σ CSS MLS SSS NOTE

PG 0824+288 0.093 13.89 13.96

PG 0825+428 0.069 15.08

PG 0832+675 0.147 14.14

PG 0833+699 BSL

PG 0836+619 0.268 18.91

PG 0837+401 0.027 15.44

PG 0838+165 0.220 15.68 15.59

PG 0838+133 0.033 13.61 13.67

PG 0839+399 0.028 14.32

PG 0839+336 0.042 14.88

PG 0841+312 0.043 14.54

PG 0845+129 0.051 15.77 15.72

PG 0848+416 0.045 16.17

PG 0848+249 0.022 14.23

PG 0848+186 0.220 19.51

PG 0849+319 0.021 14.55

PG 0850+170 0.074 13.93 14.06

PG 0850+192 0.066 16.22 16.32

PG 0854+385 0.043 15.66

PG 0856+121 0.019 13.45

PG 0900+401 0.080 12.8

PG 0901+309 0.517 20.58

PG 0902+058 0.029 14.06

34 34

Table 6. (cont.)

IDENTIFIER σ CSS MLS SSS NOTE

PG 0902+124 0.025 14.62 14.66

PG 0903-032 0.066 15.43

PG 0904+735 BSL

PG 0905+627 0.041 16.32

PG 0906+191 0.061 15.89 16.01

PG 0906+597 0.026 15.39

PG 0907+123 0.051 13.95 14.02

PG 0908+281 0.065 16.99

PG 0909+169 0.054 15.29 15.35

PG 0909+164 0.041 13.82 13.83

PG 0909+275 0.128 12.21

PG 0911+042 0.023 15.42

PG 0911+456 0.026 14.73

PG 0912+189 0.049 15.87 15.97

PG 0912+119 0.162 15.56 15.97

PG 0914+201 0.052 16.44 16.48

PG 0914+001 0.015 14.15

PG 0914-037 0.024 15.28

PG 0914+120 0.047 16.34 16.34

PG 0917+037 0.096 16.86

PG 0918+029 0.019 13.29

PG 0919+272 0.077 12.61

PG 0920+297 0.025 14.71

35 35

Table 6. (cont.)

IDENTIFIER σ CSS MLS SSS NOTE

PG 0920+029 0.129 14.33

PG 0921+208 0.035 14.65

PG 0921+311 0.289 19.37

PG 0921+161 0.048 14.56 14.64

PG 0922+259 0.020 15.42

PG 0923+329 0.399 20.29

SDSS J092859.40+061725.6 0.166 18.55

PG 0926+527 0.055 16.66

PG 0927+311 0.049 14.92

PG 0928+031 0.028 14.98

PG 0930+085 0.574 21.86 SD

PG 0931+691 0.125 16.63

PG 0932+314 0.030 15.69

PG 0932+231 0.044 16.28 16.33

PG 0932+166 0.026 14.72 14.71

PG 0933+026 0.059 15.45

PG 0933+004 0.030 14.29

PG 0934+163 0.053 16.57 16.61

PG 0934+186 0.035 13.13

PG 0934+554 0.560 12.14

PG 0935+084 0.045 15.16 15.22

PG 0935-038 0.032 14.73 14.68

PG 0935+087 0.118 16.54 16.58

36 36

Table 6. (cont.)

IDENTIFIER σ CSS MLS SSS NOTE

PG 0936+037 0.097 13.13

PG 0942+461 0.028 14.26

PG 0942-029 0.077 13.97 14.18

PG 0943+489 0.276 18.46

PG 0943+043 0.525 20.72 SD

PG 0946+305 0.286 20.46 SD

PG 0947+462 0.023 14.77

PG 0947+036 0.068 17.07

PG 0948+533 0.070 15.31

PG 0948+041 0.544 20.51 SD

PG 0948+632 0.047 14.81

PG 0949-101 0.038 14.53 14.16

PG 0950+185 0.019 14.81

PG 0950+158 0.038 15.52 15.56

PG 0950+120 0.357 22.29 SD

PG 0952+518 0.095 12.72

PG 0953+024 0.021 14.98

PG 0954+049 0.084 12.92

PG 0954+247 0.040 15.53 15.64

PG 0956+045 0.027 15.75

PG 0956-117 0.059 15.56 15.77

PG 0956+35 0.040 15.34

PG 0957+037 0.025 15.44

37 37

Table 6. (cont.)

IDENTIFIER σ CSS MLS SSS NOTE

PG 0958-116 0.057 15.21 15.41

PG 0958-119 0.044 14.04

PG 0958-073 0.043 13.55 13.61

PG 0959-085 0.058 16.25

PG 0959+297 0.045 16.36

PG 1000+375 0.030 14.92

PG 1000+408 0.054 13.31

PG 1006-145 0.055 15.14 15.05

PG 1008+756 BSL

PG 1009+491 0.045 16.51

PG 1011+649 0.047 14.91

PG 1012+007 0.020 14.87

PG 1017-113 0.060 16.4 16.46

PG 1017+430 0.028 15.31

PG 1017-086 0.125 14.44 14.44

PG 1018-047 0.083 13.27 13.46

PG 1020+694 BSL

PG 1020+142 0.095 15.06 15.05

PG 1021-029 0.103 15.41 15.66

PG 1022+459 0.034 15.88

PG 1024+238 ML

PG 1025+244 0.358 19.78

PG 1025+258 0.029 16.22

38 38

Table 6. (cont.)

IDENTIFIER σ CSS MLS SSS NOTE

PG 1026+002 0.020 13.62

PG 1026-037 0.036 15.93 15.97

SDSS J102907.31+254008.3 0.074 17.07

PG 1027-077 0.076 16.2

PG 1029-048 0.051 16.45 16.47

PG 1030-104 0.057 16.61

PG 1030+665 0.037 15.51

PG 1032+40 0.135 11.58

PG 1033+201 0.036 15.6 15.63

PG 1036+433 0.141 11.18

PG 1037+143 0.079 14.53 14.53

PG 1038+510 0.085 15.03

PG 1038+139 0.041 15.72 15.82

PG 1039-118 NAOF

PG 1039+219 0.031 13.1

PG 1040+451 0.054 16.98

PG 1040+234 0.039 13.34

PG 1043+760 BSL

PG 1040+234 0.039 13.34

PG 1043+760 BSL

PG 1043+206 0.046 16.24

PG 1045+100 0.069 17.03

PG 1046+189 0.017 14.94

39 39

Table 6. (cont.)

IDENTIFIER σ CSS MLS SSS NOTE

PG 1047+003 0.044 13.44

PG 1047-046 0.037 15 15.06

PG 1047-066 0.067 14.75 14.89

PG 1049+103 0.048 15.3 15.39

PG 1049+013 0.077 14.11

PG 1050-065 0.032 14.2 14.26

PG 1051+501 0.030 13.39

PG 1052-081 0.047 16.27 16.21

PG 1056+324 0.026 14.63

PG 1100+526 0.075 17.11

PG 1100-008 0.123 16.46 16.68

PG 1100-141 ML

PG 1100+591 0.032 13.45

PG 1101+529 0.024 14.84

PG 1101+113 0.091 15.11 15.08

PG 1101+385 0.319 12.01

PG 1101+249 0.071 12.76

PG 1102+097 0.069 16.26

PG 1102+499 0.032 14.33

PG 1104+022 0.056 14.67 14.52

PG 1104+299 0.029 15.74

PG 1104+243 0.118 11.24

PG 1105+298 0.029 15.51 15.55

40 40

Table 6. (cont.)

IDENTIFIER σ CSS MLS SSS NOTE

PG 1106+083 0.069 15.98 16.1

PG 1106+608 0.029 14.83

PG 1106-097 0.103 16.48 16.61

PG 1106+271 0.021 15.5

PG 1108-018 0.039 16.51

PG 1109-070 0.017 14.39

PG 1109-016 0.021 15.27

PG 1110+294 0.020 14.09

PG 1110+045 NAOF

PG 1111+204 0.198 22.63

PG 1111-077 0.071 13.71

PG 1111+339 0.117 12.29

PG 1112+339 0.040 15.21

PG 1114+072 0.080 13.05 13.03

PG 1115-065 0.034 15.01 14.96

PG 1115+275 0.031 15.96

PG 1116+301 0.028 14.32 14.39

PG 1116+349 0.026 13.39

PG 1117+312 0.041 14.54

PG 1118+099 0.069 16.46 14.48

PG 1118+061 0.041 14.19 14.13

PG 1119+377 0.031 15.93

PG 1119+147 0.060 16 15.86

41 41

Table 6. (cont.)

IDENTIFIER σ CSS MLS SSS NOTE

PG 1119+619 0.022 15.48

PG 1122+517 0.022 16.46

PG 1124+123 NAOF

PG 1125+175 NAOF

PG 1125-055 0.069 16.82 16.81

PG 1125+295 0.030 15.09

PG 1126+185 0.031 13.74

PG 1127-088 0.060 16.72 16.66

PG 1127+019 0.043 13.79 13.87

PG 1127+746 BSL

PG 1128+098 0.028 14.33 14.29

PG 1129-081 0.178 18.38 18.37

PG 1130-063 0.057 16.17 16.24

PG 1130+054 0.053 14.79 14.8

PG 1130+564 0.022 15.28

PG 1131+614 0.026 15.4

PG 1133+103 0.099 15.48 15.48

PG 1133+489 0.077 17.08

PG 1134+463 0.054 15.24

PG 1134+144 0.031 13.08

PG 1135+585 NAOF

PG 1135-116 0.059 15.89 15.85

PG 1136-003 0.050 14.45 14.51

42 42

Table 6. (cont.)

IDENTIFIER σ CSS MLS SSS NOTE

PG 1137+470 NAOF

PG 1138+425 0.045 15.83

PG 1138-101 0.039 14.54 14.46

PG 1140-050 0.042 14.99 14.99 15.13

PG 1140-078 0.101 16.79 16.71

PG 1141-116 0.031 14.39 14.39

PG 1141+000 NAOF

PG 1142-037 0.023 15.9 15.88

PG 1144+005 0.040 15.19 15.22

PG 1144+615 0.025 13.5

PG 1145+446 0.031 15.76

PG 1145-135 0.071 14.32 14.33

PG 1146+228 0.033 14.87

PG 1146+722 BSL

PG 1147-085 0.092 16.66 16.64

PG 1149+394 0.028 15.34

PG 1150-105 0.054 14.99 14.91

PG 1151+359 0.037 16.54

PG 1152-119 0.062 16 15.97

PG 1153-119 0.088 16.84 16.76

PG 1153+344 0.023 14.85

PG 1153-080 0.065 16.35

PG 1154-070 0.064 14.3 14.37

43 43

Table 6. (cont.)

IDENTIFIER σ CSS MLS SSS NOTE

PG 1155+741 BSL

PG 1156-037 0.040 13.37 13.47

PG 1157+004 0.042 15.68

PG 1200+094 BSL

PG 1201+258 0.473 20.34

PG 1201+436 BSL

PG 1202+608 0.018 13.58

PG 1202+404 BSL

PG 1203+093

PG 1203-108 0.049 15.87 15.86

PG 1203+084 0.049 16.18

PG 1203+574 BSL

PG 1204+543 0.033 16.02

PG 1206+373 0.027 15.04

PG 1206+165 0.019 13.64

PG 1208+224 0.069 15.06

PG 1210+141 0.029 14.7

PG 1210+429 0.018 14.96

PG 1210+464 0.023 14.22

PG 1212+369 0.073 13.12

PG 1214+031 0.073 13.86 14.03

PG 1215+025 0.082 16.84

PG 1217-067 0.057 15.17 15.21 15.29

44 44

Table 6. (cont.)

IDENTIFIER σ CSS MLS SSS NOTE

PG 1219+534 0.092 13.28

PG 1220-056 0.069 14.69 14.8 14.88

PG 1223+059 0.061 16.3

PG 1223+617 0.331 16.9 SD

PG 1224+672 0.130 11.92 SD

PG 1225-122 0.045 14.56 14.63

PG 1226-107 0.106 16.53 16.74

PG 1230+052 0.077 13.18 13.37

PG 1230+067 0.039 13.21 13.24

PG 1232-136 0.116 13.27 13.37

PG 1232+229 0.040 15.58

PG 1233+426 0.098 11.94

PG 1234+481 0.023 14.43

PG 1234+505 0.035 14.71

PG 1237+132 0.014 14.57

PG 1237-141 0.061 16.24 16.3

PG 1237+118 0.038 15.98

PG 1237+233 0.018 14.99

PG 1238+515 0.047 13.68

PG 1239+178 0.171 11.76

PG 1239+044 0.026 15.54 15.52

PG 1240+120 0.108 15.68

PG 1241-101 0.125 15.57 15.7

45 45

Table 6. (cont.)

IDENTIFIER σ CSS MLS SSS NOTE

PG 1242-106 0.085 14.63 14.82

PG 1242+241 0.023 15.34

PG 1242+604 0.033 16.35

PG 1244-004 0.086 16.48 16.59

PG 1244+113 0.020 14.11

PG 1245+041 0.060 16.82 16.68

PG 1246-122 0.049 14.6 14.53

PG 1247-115 0.038 15.35 15.35

PG 1247+042 0.054 16.33 16.2

PG 1248+374 0.042 15.52

PG 1248+066 0.040 16.13 16.12

PG 1249+761 BSL

PG 1249+160 0.017 14.62

PG 1249-028 0.062 15.68 15.74 15.81

PG 1250+303 0.027 15.86

PG 1251+019 0.022 15.55

PG 1253+284 0.071 12.71

PG 1254+279 0.022 15.84

PG 1255+546 0.046 13.55

PG 1256+278 0.015 14.2

PG 1257+276 0.025 15.35

PG 1257+171 0.073 14.27

PG 1257-026 0.040 13.99 14.07 13.98

46 46

Table 6. (cont.)

IDENTIFIER σ CSS MLS SSS NOTE

PG 1257+010 0.080 15.71 15.84

PG 1258+012 0.075 16.46 16.56

PG 1258-030 0.068 13.14 13.21

PG 1300+278 0.019 14.25

PG 1301+270 0.023 15.66

PG 1303+122 0.026 16.05

PG 1303+097 0.024 14.44

PG 1303-114 0.083 13.66 13.82

PG 1304+491 0.023 13.83

PG 1309-078 0.031 14.07 14.07

PG 1310+179 0.110 15.33

PG 1310+548 0.040 15.88

PG 1311+372 0.021 14.55

PG 1313+165 0.028 15.79

PG 1314+041 0.037 15.73 15.83

PG 1314+442 0.157 15.25

PG 1314+003 0.038 16.04 16.03

PG 1315-123 0.081 15.05 15.06 15.22

PG 1315+645 0.018 15.02

PG 1315+013 0.050 16.88

PG 1316+678 0.034 15.96

PG 1316-125 0.062 15.19 15.18 15.29

PG 1316+449 0.170 14.87

47 47

Table 6. (cont.)

IDENTIFIER σ CSS MLS SSS NOTE

PG 1316+212 0.046 15.17

PG 1317+123 0.180 11.32

PG 1318+062 0.066 14.74

PG 1319+405 0.074 16.79

PG 1321+265 0.059 16.17

PG 1322+286 0.026 15.21

PG 1323+391 0.052 16.22

PG 1323-085 0.139 13.46 13.56 13.53

PG 1323+125 0.024 15.34

PG 1323+042 0.050 15.27 15.22

PG 1325+101 0.025 13.96

PG 1325+054 0.017 14.36

PG 1326-132 0.187 20.12

PG 1327-119 0.064 15.98 16.09 16.07

PG 1327+546 0.035 14.75

PG 1328+000 0.039 16.14

PG 1329+159 0.022 13.45

PG 1330-074 0.044 14.87 14.89

PG 1331+591 0.030 14.99

PG 1332-091 0.078 16.44 16.55 16.5

PG 1334+629 0.020 15.09

PG 1334+117 0.030 16.33

PG 1336-018 0.196 13.44 13.47

48 48

Table 6. (cont.)

IDENTIFIER σ CSS MLS SSS NOTE

PG 1338+481 0.044 13.65

PG 1338+611 0.145 11.71

PG 1339+052 0.040 16.27

PG 1340+607 0.026 12.8

PG 1343-101 0.064 13.69 13.79 13.7

PG 1344+285 0.032 14.88

PG 1344+114 0.026 14.97

PG 1347+086 0.101 11.67

PG 1348+745 BSL

PG 1348+083 0.030 15.73

PG 1348+606 0.053 16.36

PG 1348+369 0.106 13.41

PG 1349+012 0.032 15.65 15.78

49 49 List of PG Hot Subdwarf Stars Part II

Table 7: PG Standard Stars from Figure 6.

IDENTIFIER 휎 CSS MLS SSS NOTE

PG 1349+659 0.030 16.02

PG 1350+372 0.048 14.34

PG 1351+393 0.046 14.28

PG 1351+139 0.035 15.98

PG 1352-022 0.105 12.03

PG 1352+119 0.031 15.5

PG 1353+162 0.032 15.91

PG 1353+152 0.031 15.98

PG 1355-064 0.037 13.66 13.7 13.61

PG 1355+071 0.032 14.26

PG 1356-047 0.070 16.11 16.2

PG 1356+242 0.031 15.04

PG 1356+354 0.023 14.87

PG 1357+501 0.035 16.07

PG 1357+239 0.022 15.16

PG 1359+077 0.027 15.99

PG 1359+003 0.054 16.65 16.66

PG 1400+389 0.193 12.18

PG 1400+395 0.165 15.61

PG 1400+224 0.026 15.8

PG 1401+289 0.026 14.89

PG 1401+377 0.153 16.26 50 50

Table 7. (cont.) IDENTIFIER 휎 CSS MLS SSS NOTE PG 1402+065 0.067 15.61

PG 1402+251 0.027 14.65

PG 1403+019 0.030 15.81

PG 1403-111 0.084 15.29 15.3 15.37

PG 1403+070 0.019 14.28

PG 1403-110 0.315 22

PG 1403+316 0.025 13.52

PG 1405+241 0.041 16.11

PG 1407+387 0.153 15.97

PG 1407+005 0.032 15.63

PG 1407-013 0.022 13.73 13.78

PG 1408+326 0.033 14.93

PG 1408+098 0.018 14.05

PG 1409+604 NAOF

PG 1409-103 0.055 14.18 14.18 14.07

PG 1409-091 0.027 14.4 14.43 14.39

PG 1411+219 0.412 14.47

PG 1411+218 0.047 16.05

PG 1411+59 NAOF

PG 1411-061 0.024 14.58 14.54

PG 1412+299 0.050 16.64

PG 1412+612 0.018 14.9

PG 1413+421 0.025 14.83 51 51

Table 7. (cont.) IDENTIFIER 휎 CSS MLS SSS NOTE PG 1413+015 0.337 17.1 17.2

PG 1413+114 0.036 16.03

PG 1415-043 0.022 13.75 13.78

PG 1415+492 0.037 14.33

PG 1415+079 0.036 15.68

PG 1416+11 0.039 16.49

PG 1417+257 0.051 13.77

PG 1418+178 0.029 16.32

PG 1419+081 0.707 14.99

PG 1420+162 0.017 15.31

PG 1420+518 0.299 15.55

PG 1421+15 0.025 15.32

PG 1421-121 0.051 15.44 15.44 15.37

PG 1421+345 0.023 14.75

PG 1422+035 0.034 16.47

PG 1423-013 0.037 16.57

PG 1423-035 NAOF

PG 1424+332 0.032 14.08

PG 1425+019 0.043 16.15

PG 1425+590 0.057 16.09

PG 1425+219 0.024 15.49

PG 1426+047 0.051 16.79

PG 1426+213 0.016 13.17 52 52

Table 7. (cont.) IDENTIFIER 휎 CSS MLS SSS NOTE PG 1426-067 NAOF

PG 1427+195 0.025 14.09

PG 1427+283 0.045 15.52

PG 1428+513 0.054 16.89

PG 1430+066 0.037 15.88

PG 1430-083 0.093 15.9 15.94 15.88

PG 1431+081 0.042 16.66

PG 1431-079 0.201 19.94

PG 1432+091 0.033 16.38

PG 1432+004 0.075 12.71 12.83

PG 1432+108 0.028 16.19

PG 1432+158 0.058 13.89

PG 1433+239 0.088 12.51

PG 1434+386 0.038 15.21

PG 1435+098 0.025 15.81

PG 1437+727 BSL

PG 1437+233 0.034 14.91

PG 1438-078 0.041 14.28 14.26 14.23

PG 1438+056 0.060 16.47

PG 1438+056 0.061 16.47

PG 1439-013 0.027 13.89 13.96

PG 1440+174 0.035 15.49

PG 1442+346 0.035 16.08 53 53

Table 7. (cont.) IDENTIFIER 휎 CSS MLS SSS NOTE PG 1442+144 0.073 16.4

PG 1443+177 0.402 20.65

PG 1443+336 0.087 15.82

PG 1444+487 0.029 15.73

PG 1444+076 0.017 14.67

PG 1444+236 0.040 13.09

PG 1445+584 BSL

PG 1446+084 0.070 16.37

PG 1446+088 0.043 16.19

PG 1447+176 0.048 16.74

PG 1447+459 0.071 14.98

PG 1448-052 0.023 14.54

PG 1448+485 0.044 16.14

PG 1449+653 0.041 13.61

PG 1449+530 0.053 15.3

PG 1449+583 BSL

PG 1451+528 0.061 16.64

PG 1451+397 0.078 16.09

PG 1451+473 NAOF

PG 1452+198 0.075 12.27

PG 1453-081 0.035 14.22 14.25 14.2

PG 1453-113 0.045 15.38 15.38 15.35

PG 1454+502 0.054 16.67 54 54

Table 7. (cont.) IDENTIFIER 휎 CSS MLS SSS NOTE PG 1454+358 0.109 16.62

PG 1455-069 0.069 16.32 16.37 16.25

PG 1455+501 0.027 15.28

PG 1455+369 0.065 17.28

PG 1457+193 0.018 14.22

PG 1457+198 0.045 15.65

PG 1458+423 0.063 13.9

PG 1459-048 0.034 13.29 13.3

PG 1459-026 0.029 14.74 14.7

PG 1500+053 0.022 15.65

PG 1501+664 0.039 16.38

PG 1501+426 0.046 14.66

PG 1502+129 0.024 15.48

PG 1502+061 0.021 15.17

PG 1502-103 0.059 15.42 15.4 15.33

PG 1502+113 0.025 15.36

PG 1504+295 0.044 16.38

PG 1505+07 0.081 12.33

PG 1506+757 BSL

PG 1506-052 0.025 13.84 13.86

PG 1507-015 0.040 16.46 16.51

PG 1508+443 0.036 14.97

PG 1508+257 0.036 16.15 55 55

Table 7. (cont.) IDENTIFIER 휎 CSS MLS SSS NOTE PG 1508-101 ML

PG 1508+177 0.087 16.14

PG 1509-083 0.048 14.59 14.54 14.5

PG 1511+266 0.039 16.51

PG 1511+447 0.029 15.6

PG 1511+624 0.056 14.45

PG 1511-110 0.053 14.91 14.84 14.83

PG 1511-110 0.052 14.91 14.84 14.83

PG 1512-035 0.041 16.22 16.22

PG 1512+244 0.026 13.15

PG 1513-045 0.030 15.69 15.71

PG 1514+455 0.031 15.12

PG 1514+231 0.025 15.06

PG 1514+422 0.049 16.41

PG 1514+034 0.020 13.92 13.91

PG 1515+232 0.036 16.41

PG 1515+044 0.023 15.31 15.27

PG 1516+205 0.335 19.5

PG 1517+265 0.030 15.86

PG 1517-078 0.184 14.87

PG 1518-098 0.030 13.61 13.65

PG 1518+299 0.091 15.7

PG 1519-071 0.109 15.88 15.84 15.79 56 56

Table 7. (cont.) IDENTIFIER 휎 CSS MLS SSS NOTE PG 1519+383 0.033 15.77

PG 1519+640 0.090 12.46

PG 1520-050 0.065 15.58 15.63 15.57

SDSS J152438.12 0.027 14.91 14.91 -012830.8

PG 1522+306 NAOF

PG 1522+122 0.031 16.21

PG 1522-104 ML

PG 1524+43 0.023 15.12

PG 1524+611 0.077 12.73

PG 1525-009 0.030 14.66 14.69

PG 1525+103 0.027 15.65

PG 1525-071 0.266 14.69 15.01 14.86

PG 1525+024 0.034 15.33 15.38

PG 1525+107 0.038 15.92

PG 1526+132 0.022 14.11

PG 1526+440 0.026 15.61

PG 1527-054 0.091 16.11 16.13

PG 1528+062 0.085 14.73

PG 1528+029 0.037 15.4

PG 1528+025 ML

PG 1528+104 0.031 13.59

PG 1530+431 0.043 15.25 57 57

Table 7. (cont.) IDENTIFIER 휎 CSS MLS SSS NOTE PG 1530+459 0.037 16.51

PG 1530+057 0.035 14.09

PG 1531+277 0.027 15.57

PG 1531+447 NAOF

PG 1532+523 0.017 14.05

PG 1532-072 0.046 16.17 16.18 16.18

PG 1533+320 0.035 16.01

PG 1534+389 0.073 16.93

PG 1536+279 0.040 13.69

PG 1536+097 0.030 15.7

PG 1536+690 0.042 14.69

2MASS J153924 0.019 14.75 43+0933283

PG 1537+272 0.064 16.58

PG 1538+002 0.029 15.34 15.32

PG 1538+611 0.018 14.31

PG 1538+401 0.053 13.25

PG 1539+292 0.017 14.56

PG 1539+043 0.028 15.86 15.9

PG 1539+043 0.028 15.86 15.9

PG 1543+454 0.050 16.28

PG 1543-124 0.044 16.37 16.41

PG 1543+629 0.023 14.84 58 58

Table 7. (cont.) IDENTIFIER 휎 CSS MLS SSS NOTE PG 1544+277 0.040 16.4

PG 1544+253 0.032 14.05

PG 1544+601 0.022 14.48

PG 1544+107 0.016 14.4

PG 1544+488 0.068 12.85

PG 1545+035 0.022 14.25 14.21

PG 1546+045 0.024 15.43

PG 1547+476 0.026 15.59

PG 1547+632 0.031 14.99

PG 1548+166 0.062 14.96

PG 1549+476 0.029 15.97

PG 1551-076 0.085 15.33

PG 1551+256 0.028 13.76

PG 1551+015 0.021 15.22 15.23

PG 1552+141 0.048 16.73

PG 1553-077 0.058 14.7 14.69

PG 1553+273 0.033 13.57

PG 1554+408 0.099 16.34

PG 1554+222 0.052 13.8

PG 1554+505 0.029 16.2

PG 1555+504 0.030 16.17

PG 1555+142 0.033 15.35

PG 1555+303 0.026 14.74 59 59

Table 7. (cont.) IDENTIFIER 휎 CSS MLS SSS NOTE PG 1555+489 0.040 16.51

PG 1559+048 0.022 14.36

PG 1559+222 0.044 14.83

PG 1559+076 0.015 14.47

PG 1559+533 0.019 14.33

PG 1600+171 0.067 16.86

PG 1601+145 0.044 14.44

PG 1602+013 ML

PG 1602+149 0.053 16.1

PG 1604+504 0.027 13.02

PG 1605+072 0.101 12.85

PG 1605+123 0.029 15.28

PG 1606+627 0.025 15.57

PG 1606+387 0.110 16.64

PG 1607+227 0.060 16.3

PG 1607+174 0.104 12.17

PG 1608-029 0.202 15.64

PG 1608+373 0.052 16.85

PG 1608+481 0.059 16.26

PG 1608+443 0.039 14.89

PG 1609+013 ML

PG 1609+195 0.064 15.88

PG 1610+529 0.058 12.87 60 60

Table 7. (cont.) IDENTIFIER 휎 CSS MLS SSS NOTE PG 1610+043 0.027 16.03

PG 1610+273 0.045 16.31

PG 1610+519 0.017 13.75

PG 1610+115 0.027 14.79

PG 1611+090 0.094 14.63 14.68

PG 1611+041 0.067 15.57

PG 1612+437 0.047 16.57

PG 1612+112 0.158 17.21 17.17

PG 1612+736 BSL

PG 1613+426 0.043 14.44

PG 1613+467 0.025 14.64

PG 1613+729 BSL

PG 1614+378 0.074 17.08

PG 1614+146 0.030 14.36

PG 1615+413 0.057 17.06

PG 1616+144 0.032 13.49 13.53

PG 1617-009 0.026 15.12

PG 1617+076 NAOF

PG 1617+309 0.034 15.65

PG 1617+150 0.046 14.67 14.67

PG 1618+216 0.039 16.02

PG 1619+522 0.034 13.3

PG 1620+017 0.055 15.52 15.58 61 61

Table 7. (cont.) IDENTIFIER 휎 CSS MLS SSS NOTE PG 1621+249 0.058 15.69

PG 1621+126 0.057 15.79

PG 1621+476 0.057 16.29

PG 1622+194 0.030 15.82

PG 1622+411 0.050 14.45

PG 1623+178 0.039 15.97

PG 1623+386 0.030 15.46

PG 1624+014 ML

PG 1624+085 0.100 14.89

PG 1624+382 0.046 15.7

PG 1625-034 0.160 15.11

PG 1626+471 0.034 13.81

PG 1627+006 0.024 14.95 14.91

PG 1627+017 0.085 12.82 13

PG 1627+025 ML

PG 1627+112 ML

PG 1628+553 0.044 15.77

PG 1628+530 0.024 15.67

PG 1628+181 0.081 15.39

PG 1629+081 0.121 12.59

PG 1629+466 0.033 13.87

PG 1629+18 0.042 15.95

PG 1631+001 0.075 14.37 14.33 62 62

Table 7. (cont.) IDENTIFIER 휎 CSS MLS SSS NOTE PG 1631+266 0.082 15.6

PG 1632+001 0.050 15.82

PG 1632+053 0.055 15.41 15.44

PG 1632+088 0.165 18.15

PG 1632+588 0.058 16.57

PG 1633+099 0.137 14.35 14.31

PG 1633+696 0.052 16.33

PG 1634+06 ML

PG 1634+014 ML

PG 1635+533 0.138 15.63 15.55

PG 1635+413 0.025 13.99

PG 1636+428 0.099 16.66

PG 1636+104 0.077 13.93 13.95

PG 1636+085 0.099 16.16 16.09

PG 1637+346 0.023 15.15

PG 1638+147 0.073 14.87

PG 1638+128 ML

PG 1638+076 ML

PG 1638+676 0.050 16.15

PG 1639+173 0.040 15.81

PG 1639+338 0.152 15.53

PG 1640+645 1.507 13.4

PG 1641+331 0.058 16.31 63 63

Table 7. (cont.) IDENTIFIER 휎 CSS MLS SSS NOTE PG 1642+038 0.029 15.31 15.36

PG 1642+707 BSL

PG 1643+209 0.092 15.72

PG 1644+404 0.028 14.15

PG 1644+312 0.065 14.23

PG 1645+610 0.020 14.42

PG 1646+607 0.043 16.52

PG 1646+062 0.082 15.7 15.65

PG 1646+043 0.056 15.57 15.56

PG 1646+250 NAOF

PG 1646+354 0.040 16.56

PG 1647+253 0.049 14.01

PG 1647+056 0.029 14.62 14.62

PG 1648+315 0.047 15.92

PG 1648+081

PG 1648+536 0.046 14.02

PG 1649+356 0.024 15.15

PG 1649+522 0.045 16.28

PG 1650+706 BSL

PG 1650+280 0.030 14.79

PG 1651+086 0.040 15.3 15.21

PG 1652+307 0.024 15.38

PG 1652+159 0.026 15.49 64 64

Table 7. (cont.) IDENTIFIER 휎 CSS MLS SSS NOTE PG 1652+517 0.042 16.19

PG 1653+131 0.038 14.38

PG 1653+115 0.025 14.67

PG 1653+544 0.032 15.32

PG 1653+633 0.032 16.24

PG 1654+322 0.030 15.46

PG 1655+106 0.247 15.49

PG 1656+322 0.041 14.73

PG 1656+253 0.044 16.05

PG 1656+600 0.033 16.14

PG 1656+213 0.024 14.61

PG 1656+271 0.045 16.19

PG 1656+318 0.096 14.29

PG 1656+356 0.044 16.39

PG 1656+553 0.034 15.57

PG 1657+291 0.035 16.13

PG 1657+416 0.043 16.04

PG 1657+078 0.035 14.88 14.88

PG 1657+656 0.047 16.18

PG 1658+273 0.035 15.73

PG 1658+337 0.039 15.95

PG 1659+111 0.035 15.27

PG 1659+123 0.055 16.14 65 65

Table 7. (cont.) IDENTIFIER 휎 CSS MLS SSS NOTE PG 1700+198 0.049 15.71

PG 1700+247 0.038 15.95

PG 1700+315 0.703 17.2

PG 1700+486 0.054 14.12

PG 1701+359 0.038 13.23

PG 1702+099 0.062 15.77 15.79

PG 1703+355 0.033 15.66

PG 1703+074 0.079 16.48 16.6

PG 1704+222 0.081 12.66

PG 1704+466 0.038 16.07

PG 1704+441 0.029 15.86

PG 1705+398 0.037 16.44

PG 1705+505 0.092 16.82

PG 1706+357 0.030 15.5

PG 1706+301 0.035 15.95

PG 1707+657 0.045 16.27

PG 1707+214 0.033 15.6

PG 1708+614 0.022 15.12

PG 1708+409 0.033 15.17

PG 1708+602 0.019 13.77

PG 1709+138 0.063 16.06

PG 1710+566 NAOF

PG 1710+278 0.028 15.26 66 66

Table 7. (cont.) IDENTIFIER 휎 CSS MLS SSS NOTE PG 1710+490 0.064 12.92

PG 1711+564 0.039 16.05

PG 1712+228 0.025 14.81

PG 1713+248 0.038 16.04

PG 1715+273 0.063 16.92 16.98

PG 1715+457 0.046 16.11

PG 1716+426 0.034 13.98

PG 1716+367 0.057 15.46

PG 1717+258 0.040 14.45

PG 1717+607 0.019 14.45

PG 1717+474 0.036 15.77

PG 1718+519 0.077 13.54

PG 1722+286 0.023 13.39

PG 1722+317 0.038 15.98 16.04

PG 1722+353 ML

PG 1723+603 0.030 15.53

PG 1724+523 0.058 16.08

PG 1724+590 0.024 14.63

PG 1724+278 0.070 16.03

PG 1725+373 0.024 15.54

PG 1725+285 0.069 16

PG 1725+245 0.053 16.41

PG 1725+252 0.044 12.98 13.04 67 67

Table 7. (cont.) IDENTIFIER 휎 CSS MLS SSS NOTE PG 1729+272 0.030 15.69

PG 1729+500 0.058 16.74

PG 1729+371 0.034 16.04

PG 1733+326 ML

PG 1738+505 0.022 13.23

PG 1743+477 0.028 13.78

PG 2050+001 0.070 16.69

PG 2052+027 0.058 16.12

PG 2052-003 0.047 15.13

PG 2059+013 0.030 14.99

PG 2102+037 0.027 15.2

PG 2110+001 0.076 16.37

PG 2110+127 0.078 12.86

PG 2111+023 0.036 13.22

PG 2115+145 0.035 15.04

PG 2116+008 0.051 15.79

PG 2118+126 0.039 13.46

PG 2120+062 0.043 14.32

PG 2122+081 0.123 16.59

PG 2124+071 0.042 14.01

PG 2125+098 0.109 16.67

PG 2128+096 0.040 14.75

PG 2128+089 0.042 15.5 68 68

Table 7. (cont.) IDENTIFIER 휎 CSS MLS SSS NOTE PG 2128+146 0.045 13.16

PG 2129+151 0.033 14.15

PG 2130+067 0.057 16.15

PG 2131+066 0.063 16.37

PG 2131+164 0.039 14.41

PG 2132+095 0.038 15.57

PG 2132+126 0.022 14.84

PG 2135+045 0.023 14.56

PG 2138+049 0.037 14.51

PG 2146+087 0.214 19.18

PG 2148+095 0.036 12.99

PG 2151+100 0.053 12.59

0.132 12.09

PG 2155+175 0.068 15.82

PG 2158+082 0.059 13.12

PG 2159+051 0.066 12.92

PG 2200+085 0.084 13.87

PG 2201+145 0.053 15.85

PG 2201+059 0.409 20.32

PG 2204+035 0.309 14.21

PG 2204+127 0.037 15.82

PG 2205+023 0.043 14.12

PG 2208+014 0.034 15.76 69 69

Table 7. (cont.) IDENTIFIER 휎 CSS MLS SSS NOTE PG 2208+056 ML

PG 2212+027 0.031 15.53

PG 2212+173 0.035 15.22

PG 2213-006 0.023 14.01

PG 2214+184 0.031 14.22

PG 2215+120 0.077 16.02

PG 2215+151 0.055 14.55

PG 2217+059 0.679 16.57

PG 2218+052 0.067 15.2

PG 2218+020 0.037 14.12

PG 2219+094 0.125 11.87

PG 2220+006 0.039 16.28

PG 2223+143 0.055 14.01

PG 2223+171 0.038 14.66

PG 2226+094 0.075 13.91

PG 2228+120 0.049 15.7

PG 2229+099 0.055 13.17

PG 2230+121 0.087 15.86

PG 2234+160 0.060 15.38

PG 2235+082 0.058 15.39

PG 2236+122 0.069 16.23

PG 2236+134 0.178 14.92

PG 2237+018 0.034 15.85 70 70

Table 7. (cont.) IDENTIFIER 휎 CSS MLS SSS NOTE PG 2239+043 SD

PG 2240+193 0.109 15.93

PG 2240+105 0.025 15

PG 2244+031 0.059 16.24

PG 2244+153 0.068 15.68

PG 2246+019 0.032 15.64

PG 2249+220 0.067 16

PG 2251+080 0.051 15.69

PG 2254+067 0.027 15.22

PG 2258+155 0.035 15.38

PG 2259+134 NAOF

PG 2300+166 0.076 12.87

PG 2300+158 0.201 16.39

PG 2301+259 NAOF

PG 2303+115 0.021 14.32

PG 2304+193 NAOF

PG 2306+027 NAOF

PG 2314+076 0.348 13.7

PG 2315+071 0.340 14

PG 2315+089 0.143 14.47

PG 2317+046 0.086 12.78

PG 2318+239 0.020 13.58

PG 2320+087 0.201 14.52 71 71

Table 7. (cont.) IDENTIFIER 휎 CSS MLS SSS NOTE PG 2321+142 0.038 14.69

PG 2321+214 0.051 13.65

PG 2323+121 0.038 14.92

PG 2323+049 0.043 15.36

PG 2326+297 0.039 14.72

PG 2329+232 NAOF

PG 2331+055 0.128 15.11

PG 2331+038 0.036 14.91

PG 2331+075 0.134 16.22

PG 2335+107 0.245 15.47

PG 2335+133 NAOF

PG 2336+264 0.027 14.67

PG 2337+266 0.106 15.96

PG 2337+070 0.026 14.82

PG 2339+199 0.055 16

PG 2341+184 0.033 15.74

PG 2343+267 0.044 16.42

PG 2345+318 0.027 14.18

PG 2345+241 0.097 12.33

PG 2346+149 NAOF

PG 2348+269 NAOF

PG 2349+002 0.034 13.29

PG 2350+099 NAOF 72 72

Table 7. (cont.) IDENTIFIER 휎 CSS MLS SSS NOTE PG 2352+181 0.034 13.35

PG 2352+259 0.026 14.92

PG 2354+298 NAOF

PG 2355+242 NAOF

PG 2355+082 NAOF

PG 2356+167 0.030 14.19

PG 2357+125

PG 2358+107 NAOF

PG 2359+197 0.229 15.62

RESULTS AND DISCUSSION

The 985 stars had been split in roughly half, where the first analysis had 410 stars and the second analysis had 486. Tables 6 and 7 list these objects, as well as objects that were omitted from the analysis. We designate these with “NAOF”, which means “No Astronomical Object Found,” the error message given by CRTS’s Catalina Surveys Data Release (CSDR2) web page (http://nesssi.cacr.caltech.edu/DataRelease/) whenever it could not resolve a source through the SIMBAD database. Another error message that appeared was BSL (Beyond Survey Limits), which appears whenever the CRTS does not cover the area of the sky that contains an object. In rare cases the error message ML appeared, which means (Multiple Light curves), where CRTS produced many dramatically different light curves for one star. Stars noted as ML are possibly due to multiple stars within the field of view. The stars labeled SD (Sparse Data) are those for which the photometry only had a few data points, which is too sparse to trust. Overall, 11 of the 985 stars have the highest statistically significant variability. Two of the 11 stars are already known to be variable (one is not even a star!). One of the 11 stars is actually classified as a standard star. Photometry from CRTS for each of the 11 stars follows, along with a short description on what is currently known for each star and if the stars are known to be variable in the literature.

PG 0934+554 PG 0934+554 (Figure 8) is listed as an sdO star in the PG catalog (Green et al. 1986). SIMBAD lists B = 11.90, U – B = -1.16 and B – V = –0.26. Searches with the NASA ADS database and the SIMBAD database do not show any 74 74 obvious reports of variability. Indeed, Massey et al. (1988) classify PG 0934+554 as a spectrophotometric standard star, because of its strong, blue continuum and weak spectral lines. Since this light curve clearly shows it to be variable, PG 0934+554 should not be used as a standard star. The Data Discovery Tool of the US Virtual Astronomical Observatory (usvao.org) shows that PG 0934+554 is listed in the NASA Extragalactic Database (NED) as a BL Lac object, with a recession velocity of 10,958 km/s and a redshift z = 0.036552. This light curve is consistent with the erratic light curves characteristic of BL Lac objects. BL Lac objects are also called blazars: they are a type of quasar, which are the most luminous active galactic nuclei (AGNs) known. In a BL Lac object, one of the relativistic jets shooting out of the central engine is pointed in the direction of Earth. This makes for a strong, blue continuum of synchrotron radiation, which drowns out all but the strongest spectral lines. Since these objects are at cosmological distances, their host galaxies are often difficult to resolve. Because of this and their strong, blue continua and weak lines, BL Lac objects are sometimes misclassified as hot subdwarf stars, as PG 1101+385 (Figure 9) shows.

Figure 8. Light curve of PG 0934+554. 75 75 PG 1101+385 PG 1101+385 (Figure 9) is listed as a DC COMP (composite-spectrum) star in the PG catalog (Green et al. 1986), with Bpg = 13.34. PG 1101+385 was subsequently identified as Markarian 421 (Mrk 421), well known as one of the brightest BL Lac objects in the sky. Mrk 421 undergoes non-periodic, rapid variations on time scales of hours to days (Liu et al. 1997). Note the resemblance of its erratic light curve to that of PG 0934+554, which gives us confidence that PG 0934+554 is also a BL Lac object.

Figure 9. Light curve of PG 1101+385.

PG 1025+244 PG 1025+244 (Figure 10) is listed as an sdB star in the PG catalog (Green et al. 1986), which reports Bpg = 15.99, apparently much brighter than we observe here. There is no apparent reference to variability in literature. 76 76

Figure 10. Light curve of PG 1025+244.

PG 0923+329 PG 0923+329 (Figure 11) is listed as an sdB star in the PG catalog (Green et al. 1986), with Bpg = 16.46, much brighter than shown here. There is no apparent reference to variability in literature.

PG 1201+258 PG 1201+258 (Figure 12) is listed as an sdB-O star in the PG catalog

(Green et al. 1986), with Bpg = 15.31, much brighter than shown here. There is no apparent reference to variability in literature. CSS lists a V magnitude of 20.34 (after using the transformation from unfiltered values).

77 77

Figure 11. Light curve of PG 0923+329.

Figure 12. Light curve of PG 1201+258. 78 78 PG 0901+309 PG 0901+309 (Figure 13) is listed as an sdB-O star in the PG catalog

(Green et al. 1986), with Bpg = 14.84, much brighter than shown here. There is no apparent reference to variability in literature.

Figure 13. Light curve of PG 0901+309.

PG 1640+645 PG 1640+645 (Figure 14) is classified as a sdB star in the PG catalog

(Green et al. 1986), with Bpg = 15.17. SIMBAD shows no apparent reference of variability in the literature. Notice how this object has an apparent quiescent state near V = 15, with sudden incursions to V = 10-11 and just-as-sudden declines to V = 15. This behavior is characteristic of a dwarf nova, a class of cataclysmic variables that have outbursts. These outbursts are caused by thermal instabilities in an accretion disk around a white dwarf. When near outburst maximum, the spectrum of a dwarf nova can resemble that of a hot subdwarf, with a bright, blue 79 79 continuum overwhelming Balmer and He I lines in either weak emission or absorption. Further spectroscopic and time-resolved photometric follow-up observations are recommended, to determine the nature of this object.

Figure 14. Light curve of PG 1640+645.

PG 1419+081 PG 1419+081 (Figure 15) is listed as a sd star in the PG catalog (Green et al. 1986), with Bpg = 15.43. In a survey for pulsating subdwarf B stars, Ostensen et al. (2010), report this star as a short-period pulsator. The V magnitude reported is 14.9 which matches well with the CSS V magnitude of 14.99. It has an amplitude maximum of 7 mma (mma is milli-modulation amplitude, a unit that is 10-3 of the Fourier amplitudes of a light-curve in normalized intensity units), and a pulsation period of 143 seconds.

80 80

Figure 15. Light curve of PG 1419+081.

PG 2217+059 PG 2217+059 (Figure 16) is listed as a sdOB star in the PG catalog (Green et al. 1986), with Bpg = 16.20, no V magnitude reported, and no apparent reference of variability in literature. Here, we see this object spends most of its time near V = 16.8, with eight observations between V = 11-14. This is consistent with this object being a dwarf nova, which can be quite blue (with B − V ~ 0-0.5) and have outbursts with typical amplitudes of 2-5 magnitudes. Further spectroscopic and time-resolved photometric follow-up observations are recommended, to determine the nature of this object. 81 81

Figure 16. Light curve of PG 2217+059.

PG 1700+315 PG 1700+315 (Figure 17) is listed as a sd star in the PG catalog (Green et al. 1986), with Bpg = 16.20, no V magnitude listed, and no apparent reference to variability in literature. The way the light curve clearly follows two distinctive levels, at V ~ 16.8 and V ~ 18.7, is the behavior characteristic of an eclipsing binary star system. A period search with CRTS does not reveal a distinctive period, which suggests this is a short-period eclipsing binary, with an orbital period less than the average time between observations, which is 6.9 days for this object, with 441 CRTS observations measured over 3029.8 days. Detailed time- resolved photometric observations are recommended for this object, to search for eclipses and measure the system’s orbital period, if they are present. 82 82

Figure 17. Light curve of PG 1700+315.

PG 1411+219 PG 1411+219 (Figure 18) is listed as a DB 4 star in the PG catalog (Green et al. 1986), with Bpg = 16.20 and no apparent reference of variability in literature. Green et al. (1986) also report V = 14.38. The CSS lists V = 14.47. Here, we see this object spend most of its time near V = 14.7, with a few observations between V = 13.2. This is consistent with this object being a dwarf nova, which can be quite blue (with B − V ~ 0-0.5) and have outbursts with amplitudes between 2-5 magnitudes. That PG 1411+219 also appears to fade to fainter than V = 15 may indicate eclipses. This is of particular interest, seeing as this object has a helium spectrum (with its classification as a DB star). Is it an accreting double white dwarf, also called an AM CVn star? Further spectroscopic and time-resolved 83 83 photometric follow-up observations are recommended, to determine the nature of this object.

Figure 18. Light curve of PG 1411+219.

CONCLUSION

We present a survey of variability in hot subdwarf stars from the Palomar- Green survey, using the photometry provided by the CRTS. The 11 objects found to have the most significant variability are listed in Table 8. For each of these objects, Table 8 includes their location, mean V magnitude measured from the object’s CRTS light curve, and standard deviation of the V magnitudes measured in the object’s CRTS light curve, and whether they were previously known to be variable. Visual inspection of the light curves of these stars can provide hints into the nature of the variability. PG 1101+385 and PG 1419+081 are the only two of the 11 candidates that are previously known to be variable. PG 1101+385 was originally classified as “DC” in the PG survey. From the spectral types from the Introduction, DC classified stars are white dwarf stars. The PG 1101+385 is actually not a star, but a BL Lac-type object, which is an active galaxy. BL Lac objects can easily be misclassified as DC stars, since both have strong, blue continua that often overwhelms all but the strongest spectral lines. The other known variable star is PG 1419+081. This star was classified as an sd star in the PG catalog. It was determined to be a pulsator with a period of 143 seconds and a maximum amplitude of 7 millimagnitudes. The light curves of the other nine variable objects may provide hints as to what kind of variability they exhibit. PG 0934+554, PG 1700+315, PG 1640+645, PG 2217+059, and PG 1411+219 all have visually distinctive light curves. PG 0934+554 was identified by Massey et al. (1988) as a spectrophotometric standard star. However, according to the NASA Extragalactic Database (NED), it has the spectrum of a BL Lac object with a recession velocity of 10,958 km/s and a redshift z = 0.036552. Our 85 85 erratically variable light curve, which is typical for BL Lac objects, confirms this is a BL Lac object. This is also supported by the precision to which the position of PG 0934+554 is known (see Table 8). PG 1700+315 could be an eclipsing binary star system. Now and then the magnitude of the star will drop, suggesting the companion star has crossed in front of the other star, blocking light, and therefore we see dips in the light curve. More closely spaced photometry will be needed for these objects, as the CRTS takes data roughly every ten days. After obtaining new light curves, a period algorithm can be applied to search for the period of these systems. PG 1640+645 and PG 2217+059 may be dwarf novae. A dwarf nova is a type of cataclysmic variable star (CV), where outbursts occur due to the instability in the accretion disk. This is suggested because of their irregular brightenings in apparent magnitude, dropping back down to the quiescent state. PG 1411+219 is similar photometrically, but it also has a DB 4 helium spectrum. It may therefore be a helium CV, also known as an AM CVn star. The other candidates do not have such obvious visual clues and their variability type remains to be determined. An observation program for future students would be necessary to determine the true variability of these candidates. The program would obtain good quality light curves of the candidates and possibly spectra when needed. Intrinsic and extrinsic variability leads to many different interesting possibilities. As we have seen, a careful look at the list of hot subdwarf stars and related objects has already led us to suspicions of active galactic nuclei, cataclysmic variables, pulsating stars, and eclipsing binary star systems.

86 86 Table 8: The candidates PG 훼 훿 < 푉 > 휎 Note Identifier 0901+309 09 04 22.0 +30 45 24 20.58 0.517 Standard Star 0923+329 09 26 39.0 +32 45 30 20.29 0.399 -- +55 05 12.14 0.560 -- 0934+554 09 38 20.35359 50.0809 1025+244 10 27 54.0 +24 11 00 19.78 0.358 -- +38 12 12.01 0.319 Mrk 421 1101+385 11 04 27.31394 31.7991 1201+258 12 03 48.0 +25 32 00 20.34 0.473 -- 1411+219 14 13 29.85 +21 37 39.6 14.47 0.412 -- 1419+081 14 21 38.18 +07 53 19.6 14.99 0.707 Pulsator 1640+645 16 40 50.70 +64 24 45.3 13.4 1.507 -- 1700+315 17 02 41.542 +31 25 18.66 17.2 0.703 -- 2217+059 22 20 28.118 +06 07 22.82 16.57 0.679 --

REFERENCES

Bessel, M. S., Brett, J. M. 1988, Publications of the Astronomical Society of the Pacific, vol. 100, p. 1134

Brown, T.M., Smith, E., Ferguson, H.C., Sweigart, A.V., Kimble, R.A., Bowers, C.W. 2008, The Astrophysical Journal, vol. 682, p. 319

Brown, T.M., Ferguson, H.C., Davidsen, A.F., Dorman, B. 1997, The Astrophysical Journal, vol. 482, p. 685

Boehm-Vitense, E. 1989, Introduction to Stellar Astrophysics, vol 1. p. 185.

Carrol, B., Ostile, D. 2007, An Introduction to Modern Astrophysics 2nd Ed., p. 491

Charpinet, S., Fontaine, G., Brassard, P., Dorman, B. 1996, The Astrophysical Journal, vol. 471, p. L103

Cutri, R.M., Skrutskie, M. F., Van Dyk, S., Beichman, C. A., Carpenter, J. M., Chester, T., Cambresy, L., Evans, T., Fowler, J., Gizis, J., Howard, E., Huchra, J., Jarrett, T., Kopan, E.L., Kirkpatrick, J. D., Light, R. M., Marsh, K. A., McCallon, H., Schneider, S., Stiening, R., Sykes, M., Weinberg, M., Wheaton, W. A., Wheelock, S., Zacarias, N. 2003, The IRSA 2MASS All-Sky Point Source Catalog, NASA/IPAC Infrared Science Archive. http://irsa.ipac.caltech.edu/applications/Gator/.

Drake, A.J., Djorgovski, S. G., Mahabal, A., Beshore, E., Larson, S., Graham, M.J., Williams, R., Christensen, E., Catelan, M., Boattini, A., Gibbs, A., Hill, R., Kowalski, R. 2009, The Astrophysical Journal, vol. 696, p. 870

Green, E.M., Fontaine, G., Reed, M.D., Callerame, K., Seitenzahl, I.R., et al. 2003, The Astrophysical Journal, vol. 583, p. L31

Green, R.F., Schmidt, M., Liebert, J. 1986, The Astrophysical Journal Supplement, vol. 61, p. 305

Han, Z, Podsiadlowski, P., Maxted, P.F.L., Marsh, T.R. 2003, Monthly Notices of the Royal Astronomical Society, vol. 341, p. 669

Han, Z., Podsiadlowski, P., Maxted, P.F.L., Marsh, T.R., Ivanova, N. 2002, Monthly Notices of the Royal Astronomical Society, vol. 336, p. 449

Heber, U. 1986, Astronomy & Astrophysics, vol. 155, p. 33 88 88 Heber, U., Hunger, K., Jonas, G., Kudritzki, R.P. 1984, Astronomy & Astrophysics, vol. 130, p. 119

Heber, U. 2009, Annual Review of Astronomy & Astrophysics, vol. 47, p. 211

Kilkenny, D., Koen, C., O’Donoghue, D., Stobie, R.S. 1997, Monthly Notices of the Royal Astronomical Society, vol. 285, p. 640

Kilkenny, D. 2007, Communications in Asteroseismology, vol. 150, p. 234

Oreiro Rey, R., et al. 2004, Lecture Notes and Essays in Astrophysics, vol. 1, p. 133

Landolt, A.U. 1992a, The Astronomical Journal, vol. 104, p. 340

Landolt, A.U. 1992b, The Astronomical Journal, vol. 104, p. 372

Landolt, A.U. 2009, The Astronomical Journal, vol. 137, p. 4186

Liu, F.-K., Liu, B.-F., Xie, G.-Z. 1997, Astronomy & Astrophysics Supplement, vol. 123. 569

Massey, P., Strobel, K., Barnes, J.V., Anderson, E. 1988, The Astrophysical Journal, vol. 328, p. 315

Paczynski, B. 1971, Annual Review of Astronomy & Astrophysics, vol. 9, p. 183

Paczynski, B. 1976, in Structure and Evolution of Close Binaries, ed. P. P. Eggleton, S. Mitton & J. Whelan (Dordrecht, Kluwer), p. 75.

Saio, H., Jeffery, S. 2000, Monthly Notices of the Royal Astronomical Society, vol. 313, p. 671.

Schneider, S., Arny, T. 2009, Pathways To Astronomy, 2nd Ed. (McGraw-Hill Science/Engineering/Math), Unit 64

Schuh, S., Huber, J., Dreizler, S., Heber, U., O’Toole, S.J., et al. 2006, Astronomy & Astrophysics, vol. 445, p. L31

Silvotti, R., Schuh, S., Janulis, R., Solheim, J.-E., Bernabei, S., et al. 2007, Nature, vol. 449, p. 189

Sion, E. M., Greenstein, J. L., Landstreet, J., Liebert, J., Shipman, H. L., Wegner, G. 1983, The Astrophysical Journal, vol. 269, p. 253 89 89 Zoccali, M., Renzini, A., Ortolani, S., Greggio, L., Saviane, I., et al. 2003, Astronomy & Astrophysics, vol. 399, p. 931

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