PoS(SWIFT 10)042 http://pos.sissa.it/ ce. izing a variety of rare hot stars, properties of hot stars and young ntal Branch Stars. We also present ning within the SMC. onstraining the exact star formation sent UVOT photometry of open and Small Magellanic Cloud. We find that ive Commons Attribution-NonCommercial-ShareAlike Licen ∗ [email protected] [email protected] [email protected] [email protected] We present the results ofstellar our populations using ongoing the investigation Swift/UVOT telescope. intoglobular the We clusters pre and show thatincluding Post-Asymptotic UVOT Giant is Branch capable and of Extremevery Horizo character early reults of our survey ofthe stellar SMC populations has experienced in recent the bursts ofhistory star will formation depend but on c finding an effective model of the redde Speaker. ∗ Copyright owned by the author(s) under the terms of the Creat c Swift: 10 Years of Discovery, 2-5 December 2014 La Sapienza University, Rome, Italy Michael H. Siegel The Pennsylvania State University Department of Astronomy and Astrophysics E-mail: Caryl A. Gronwall The Pennsylvania State University Department of Astronomy and Astrophysics E-mail: Lea M. Z. Hagen The Pennsylvania State University Department of Astronomy and Astrophysics E-mail: Erik A. Hoversten University of North Carolina at ChapelDepartment Hill of Physics and Astronomy E-mail: Swift, UVOT and Hot Stars

PoS(SWIFT 10)042 ◦ 0, . M 0 5 . ]=+ H / Fe [ Michael H. Siegel color-magnitude diagrams for the . The color-magnitude diagrams e old globular cluster M 79 shows esponds to the extreme horizontal ter than 7000 degrees, are the final -M (triangles) stand out. The bottom 16,000 K that have extremely thin massive stars, which play fundamen- are best studied in the ultraviolet (UV), sequence. These stars play a role in the population in the NGC 2539 open cluster. ion, in the context of Swift, in Siegel et ollapse supernovae and GRBs (see, e.g., ributions from cool stars and older stellar evolution are well-understood due to co- erlaid with parameters set to nd thus too faint in the UV to be detected by l. (2011). bright stellar populations are labeled. M 79’s l modelling, the properties and underlying 2 65, and ages of 630 Myr, 1 Gyr and 5 Gyr. The solid blue line is a 0 . 10 = 0 ) M − m ( 05, . 0 Hot stars in clusters. The top panels show the optical and NUV )= V Figure 1 shows the variety of stars that are detected in the UV Hot stars, defined as those with effective temperatures grea − B ( white dwarf sequence using cooling curves from Bergeron et a (CMD) show two stellar populations – onea old clear and one blue young. horizontal Th branch (BHB).branch The (EHB), bend in helium-burning the stars BHBenvelopes corr with and will temperatures likely evolve over directly to the white dwarf E old stellar populations in the M 79 globular cluster. The UV- red giant branch (RGB) dominates the optical but is too cool a Isochrones from the Padova group (Bressan et al. 2012) are ov Swift/UVOT. By contrast, the BHB,panels EHB, show PAGB (square) the and color-magnitude AGB diagrams of a young stellar Figure 1: pious observational data and well-constrained theoretica populations is minimized. frontier in stellar evolution. While most phases of stellar astrophysics of hot stars remainal. contentious (see 2014). a discuss Of particulartal importance roles are in chemical the enrichment, astrophysicsLanger galaxy of et evolution, al. core-c 2012). Hot starswhere and their young spectral stellar energy populations distributions peak and the cont UVOT Hot Stars 1. Background PoS(SWIFT 10)042 Michael H. Siegel stinctive features seen he observational aspects are star in the rare short phase of astrophysics. In studies of ex- 90). NGC 2539, a young open t stars. It has a wide (17’) field ters, typically obtaining at least ng survey of Galactic star cluters. he descendants of EHB stars that numbers of field stars, providing ce of hot white dwarfs. Galactic midplane to be have been survey of nearby hot stars designed rare extremely hot (35,000 K) class the NUV are rare AGB manqué stars ations (Bruzual 2009). By measuring ve, it provides the empirical baseline iling their complex formation history ssive globular clusters from HST (see, ip of the white dwarf sequence (Bond nthesis models used in the ultraviolet opulations which can be studied on a ar populations while globular clusters e dataset of PAGB, AGB-M and EHB these objects not only provides infor- y gets further into the spectral energy rs, which are sometimes too large and 14), the nearby Magellanic Clouds and eeded to constrain the models of stellar udied field white dwarf stars (Siegel et nnell 1999, Sandquist et al. 2008). The ant galaxies. ir bulk properties. Finally, the 2.3 arc- s and nearby galaxies. These advantages ations can reveal the properties of the stars 3 Over the last several years, we have been performing an imagi To date, we have studied 97 open clusters and 65 globular clus Figures 2 and 3 show a sampling of the CMDs. Most show the the di In this contribution, we highlight the results of an ongoing The Swift/UVOT instrument is uniquely suited to studying ho Studying hot stars in the UV has implications beyond stellar to address multiple astrophysical issues.al. Our 2011, survey 2013), has open st and globularwill clusters soon (Siegel expand et to al. M31. 20 2. Open and Globular Clusters Open clusters generally provide informationprovide on information young on stell oldermation stellar on populations the Studying rare phasesneeded of to study stellar unresolved evolution stellar described populations abo in more dist 1 ks in each of UVOT’sdiscussed three in NUV detail filters in (uvw1, Siegel uvm2 et and al. uvw2). (2014). T in Figure 1. For thestars. globular At clusters, present, this we provides have a notof star detected uniqu that any have Blue only Hook been postively stars,e.g., identified a in Brown the et most al. ma 2012). are particularly notable in the case of Galactic open cluste of view, which enablesaccess the to study complete of stellar entiredistribution populations. of clusters Its hot and NUV stars, large sensitivit second providing resolution better allows leverage us to on resolve the stars in cluster core sparse to be effectively studiedobserved with by HST GALEX. and too close to the UVOT Hot Stars “UV upturn" seen in old elliptical galaxies (see, e.g., O’Co and the dust that comprise the unresolved populations, unve CMD also shows a Post-Asymptoticevolution from Giant the Branch tip star of (PAGB), the1997). a asymptotic These giant stars branch may to be the standard(AGB-M). t candles. The Also nature revealed of in thesehave stars envelopes is too unclear small but to theycluster, read may shows the be a t AGB young (Greggio blue & main sequence Renzini and 19 a sparse sequen tragalactic objects, the light of unresolved stellar popul (see, e.g., Hoversten et al.are limited 2011). by a However, lack the ofthe spectral empirical integrated sy data light on young of stellar nearbystar-by-star popul basis young – stellar we populations can provide –populations. the p emperical baseline n PoS(SWIFT 10)042 Michael H. Siegel ed from Swift/UVOT data. The oduced from Swift/UVOT data. The g. g. 4 A sampling of open cluster color-magnitude diagrams produc A sampling of globular cluster color-magnitude diagrams pr clusters span a broad range of age, metallicity and reddenin Figure 3: Figure 2: clusters span a broad range of age, metallicity and reddenin UVOT Hot Stars PoS(SWIFT 10)042 Michael H. Siegel sources measured in the sive program to observe ng empirical constraint on the ite. Processed files and point MC, it comprised 2200 images e of 1.8 days. The images were MC and nearly a million in the all size or differential reddening e NUV filters for a total exposure n 50 fields with a tpyical exposure XRT and UVOT. The imaging took the Padova program set to [Fe/H]=-0.5, study of the integrated light of stellar ar. g to the method outlined in Siegel et al. ) stars. However, a significant fraction of the ◦ 5 M 8 > A Hess diagram of the uvm2-uvw1 photometry for 250,000 point The Swift UVOT MAgellanic Clouds Survey (SUMAC) was an exten Many of the open clusters show clear main sequences, providi LMC. Full-color mosaic imagessource are catalog will available be at made publicly the available NASA later this webs ye (2014). We detected approximately 250,000 UV souces in the S the central regions of bothplace Magellanic between Clouds 2010 with September 26 the and Swift in 2013 150 November fields 5. with For a the typical L time exposure of time 5.4 of days. 3 For ks the intime SMC, UVOT’s of the thre survey 3 obtained ks 656 in imagesprocessed, UVOT’s i three stacked, NUV photometered filters and for calibrated a accordin total exposure tim 3. SUMAC open clusters do not showin a the clear field. main These sequence clusters duepopulations. are, to however, their very useful sm for the SUMAC survey. Overlayed forE(B-V)=.05 comparison and ages are of isochrones 20, 50, from 100 and 500 Myr.. properites of intermediate and high-mass ( Figure 4: UVOT Hot Stars PoS(SWIFT 10)042 Michael H. Siegel our SUMAC field to the ple from the SMC with com- rties of young main sequence lanic Clouds has revealed rich hat the reddening distribution ized the StarFISH star forma- m Pei et al. (1992), which does standing of the properties of hot his allows SUMAC to probe the pulations are younger and more ily reproduced by the synthetic r component stellar populations. e salient features of the CMD – arison of the SMC star formation llar populations and a very strong . n to date of the SMC’s recent star clarity. As can be seen, the UVOT HZ04. t the UVOT can provided unprece- MC show two distinct sequences – and reddening law vary over the face ris & Zaritsky 2004, herafter HZ04). ova program. Completeness and pho- istory is indeed driven by dynamical 4. urthermore, the final simulations will n unprecented opportunity to explore tory of these fields. Our initial results spectroscopic studies. While this may o study more distant unresolved stellar ing model used, to which the UV pho- ns. In particular, we are obtaining large . For the purpose of this first effort, we g simulations with a variety of reddening for young stellar population in the more 6 Figure 4 shows a Hess diagram of the complete photometric sam To probe the recent star formation history of the SMC, we util Figure 5 shows a comparison of the UVOT photometry from ten of Despite this limitation, this preliminary result shows tha The final result of this program will not only be a better under Our surveys of the star clusters of the Milky Way and the Magel UVOT Hot Stars parison stellar populations overlaid.the blue Optical plume CMDs of of hot thered main giant sequence S branch stars (RGB) from of theHowever, older younger the ste stellar RGB stars populations are (see too Har last cool 500 to be Myr detected of with star UVOT.data T formation is with sensitive to unprecedented the detail last and 500 Myr of star formation. tion history program of HZ04Isochrones to and luminosity deconvolve functions the were CMDs taken intotometric from the thei errors Pad were measured throughassumed artificial that star the reddening tests law ofnot the SMC have was a the blue "SMC law" bumpthat fro at best fit 2175 the Å. data After was numerous the "hot tests, star" we distribution found from HZ0 t first run of simulatedspecifically StarFISH photometry. the long As narrow canphotometry. blue be Figure plume seen, 6 of th shows the youngare inferred stars intriguing star-formation but – his show is thatmetal-rich more eas than work needs expected to from be previousbe done. investigations accurate, and The it po could alsotometry reflect is inadequacies particularly in sensitive. the We redden are currently runnin history to previous efforts. stars and probing the last 500 Myr of star formation in the SMC models in an effort to improve the fit and provide a better comp samples of BHB, EHB, PAGB and AGB-M stars, studying the prope distant universe, whose emission peaks in the UV. dented detail on the SMC’sbe recent run star-formation on history. the individual F fieldsof to the see how SMC. the The age, metallicity resultformation will and be a the critical mostinteractions test thorough with of the investigatio whether Milky Way its and star the Galaxy, formation as h posited by 4. Conclusion detail in their color-magnitude diagramsthe and properties of is rare providing hot a stars and young stellar populatio stars, but the creation of anpopulations. empirical baseline This with understanding which t is particularly critical PoS(SWIFT 10)042 (2011) 28. 1.5 737 liptical , (2009) 52. Michael H. Siegel c Confirmation in ApJ ieste Stellar 1.0 500 The Extragalactic , A&A , 0.5 d cores of nearby galaxies thetic photometry produced by 0.0 ure UV space instrumentation (in , in proceedings of , the salient feature of the CMD – the long ssimistic error model. ty high-resolution multi-filter NUV data sensitivity). Nevertheless, Swift/UVOT etic data does show a larger spread in color -0.5 -1.0 7 10 12 14 16 18 20 1.5 1.0 (2012) 127 Clues on the hot star content and the ultraviolet output of el 427 0.5 , 85 Flash Mixing on the White Dwarf Cooling Curve: Spectroscopi A Comprehensive Spectroscopic Analysis of DB White Dwarfs (1990) 35. PARSEC: stellar tracks and isochrones with the PAdova and TR 0.0 MNRAS 748 , , (1997) 224. PEGASE: a UV to NIR spectral evolution model of galaxies 364 Post-AGB stars as standard extragalactic candles ApJ , ApJ -0.5 , A comparison of the UVOT observational data (left) to the syn -1.0 galaxies NGC 2808 Distance Scale Evolution Code Studying the most massive stars (O- and B-type) in the crowde 20 18 14 16 10 12 [6] L. Greggio, & A. Renzini, [1] P. Bergeron, et al., [5] A G. Bruzual, [2] H. E. Bond, [3] A. Bressan, et al., [4] T. M. Brown, et al., is beyond UVOT’s capabilitiesparticular, and sub-arcsecond will resolution require and multi-filter improvedis FUV paving fut the way for future studiesfor by providing thousands high-quali of hot stars. References space likely due to cruder binning of the CMDs and an overly pe StarFISH (right) for a sample ofblue ten plume UVOT fields. – As is can well-reproduced be by seen the simulation. The synth Figure 5: UVOT Hot Stars PoS(SWIFT 10)042 , 2419 (2004) (1992) 130. 127 (2014) 131 1200 king a Faint Hot AJ (2012) 107. 395 Michael H. Siegel 148 , (1999) 603. 50 AJ ing Regions in M81 ApJ , 37 , s wift/UVOT, GALEX, and 1000 ARA&A , ARA&A , tude diagram above. Points are stellar e stellar population are more metal-rich 800 f a poor reddening model. point indicating the overall star-formation rate. luding a young burst of star formation at about 600 8 Age (Myr) 400 Evolved Stars in the Core of the Massive Globular Cluster NGC 1215. (2011) 205. 65. The Star Formation History of the Small Magellanic Cloud 725 Swift Ultraviolet/Optical Telescope Imaging of Star-form 141 144 200 The Swift UVOT Stars Survey. I. Methods and Test Clusters Faint Near-ultraviolet/Far-ultraviolet Standards from S Swift/UVOT Photometry of the Planetary Nebula WeBo 1: Unmas ApJ AJ Far-Ultraviolet Radiation from Elliptical Galaxies , AJ , , Interstellar dust from the Milky Way to the Magellanic Cloud Presupernova Evolution of Massive Single and Binary Stars 0 (2008) 2259. 0.0 1.0 0.5 The star-formation history used to produce the color-magni

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136 [M/H] Companion Star AJ SDSS Photometry 1531. and Holmberg IX [8] E. A. Hoversten, et al., [7] J. Harris, & D. Zaritsky, [9] N. Langer, [14] M. H. Siegel, et al., [15] M. H. Siegel, et al., [13] M. H. Siegel, et al., [12] E. L. Sandquist & J. M. Hess, [11] Y. C. Pei, Y. C., The simulations shows a number of interesting features, inc populationsthat are included in the CMD, with the size of the 400 Myr and a veryand young younger burst than of expected, star both formation of at which a are few likely Myr. a Th result o Figure 6: UVOT Hot Stars [10] R. W. O’Connell,