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Spectral Energy Distributions and Colours of Hot Subluminous Stars

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Spectral Energy Distributions and Colours of Hot Subluminous Stars Open Astronomy 2017; 1 Research Article Open Access Ulrich Heber*, Andreas Irrgang, and Johannes Schaffenroth Spectral energy distributions and colours of hot subluminous stars DOI: DOI Received ..; revised ..; accepted .. Abstract: Photometric surveys at optical, ultraviolet, and infrared wavelengths provide ever growing datasets as major surveys proceed. Colour-colour diagrams are useful tools to identify classes of stars and to provide large samples. Combining all photometric measurements of a star into a spectral energy distribution will allow quantitative analyses to be carried out. We demonstrate how to construct and exploit spectral energy distributions and colours for sublumious B (sdB) stars. The aim is to identify cool companions to hot subdwarfs and to determine atmospheric parameters of apparently single sdB stars as well as composite spectrum sdB binaries. We analyse two sdB stars with high-quality photometric data which serve as our benchmarks, the apparently single sdB HD 205805 and the sdB + K5 binary PG 0749+658, briefly present preliminary results for the sample of 142 sdB binaries with known orbits, and discuss future prospects from ongoing all-sky optical space- (Gaia) and ground-based (e.g. SkyMapper) as well as NIR surveys. Keywords: stars: early type -subdwarfs - Techniques: potometric 1 Introduction lite crucial information to study hot stars arose and early attempts to analyse SEDs of sdB stars were carried out by Heber et al. (1984), Heber (1986), and Aznar Cuadrado Optical photometry and spectroscopy provide the obser- & Jeffery (2001) by combining low resolution UV spec- vational basis for astronomy. Ongoing large photomet- tra from IUE with optical photometry. Colour-colour dia- ric surveys provide a huge amount of photometric mea- grams combining infrared and optical magnitudes are im- surements in several optical passbands, which can be portant tools to identify composite objects, such as sdB used to identify candidate hot subluminous stars, though stars with F/G/K companions (see e.g. Stark & Wade spectroscopy is needed for proper spectral typing. How- 2003; Green et al. 2008). ever, optical photometry is much more than a mere tar- Subdwarf B stars are core helium burning stars of get selection tool. Time-series photometry (light curves) half a solar mass and in the Hertzsprung Russell diagram are a crucial ingredient for asteroseismology of pulsating they form the extreme horizontal branch. Because radial stars and to identify eclipses, reflection effects, and ellip- velocity surveys have shown that the fraction of close bi- soidal variations, in compact binaries. Single-epoch ob- naries amongst single-lined (SB1) sdB stars is as high as servations, however, provide crucial information as well. 50 %, common envelope evolution plays an important role Spectroscopic distances rely on at least one measured ap- in the formation of sdB stars. About 30% of the sdB stars parent magnitude. Ultraviolet and infrared surveys when show composite colours, that is they have companions of combined with optical photometry allow us to construct arXiv:1712.06546v1 [astro-ph.SR] 18 Dec 2017 spectral types F, G, or K. In many cases the compan- broad spectral energy distributions (SED), which can be ions to SB1 systems have been found to be white dwarfs, used to determine e.g. the effective temperature of a star, but low mass main-sequence stars (spectral type M) and to identify an infrared excess hinting at the presence of brown dwarfs have been found as well (see Heber 2009, a cool companion, and to quantify interstellar absorption 2016, for reviews). Because it is often very difficult to from UV flux depression. clarify the nature of the companions from optical data Here we shall not address light variation but restrict alone, broad SEDs are the method of choice to constrain ourselves to colour-metric properties of hot subdwarf B the companions’ properties and constrain their nature. (sdB) stars. Several investigations of hot subdwarf stars We describe a method to construct broad SEDs by have made use of single-epoch photometry. With the ad- combining measurements in various photometric systems vent of the International Ultraviolet Explorer (IUE) satel- from the ultraviolet to the infrared in Sect. 2. Synthetic 2 SEDs and colours of sdB stars u v b y photometry for various photometric systems are calcu- 1 lated from grids of appropriate model atmospheres (Sect. 0.8 3). An objective method to derive various parameters 0.6 from observed SEDs is presented in Sect. 4 and prelim- 0.4 Filter response inary results are discussed in Sect. 5. We conclude with 0.2 2000 3000 4000 5000 8000 10000 15000 20000 30000 4000050000 an outlook. Wavelength [A]˚ 1 J H K W 1 W 2 0.8 2 Constructing observational 0.6 SEDs 0.4 0.2 2000 3000 5000 7000 10000 1500020000 30000 50000 The SEDs of the program stars were constructed from photometric measurements ranging from the ultraviolet Fig. 1. Response function for optical (Strömgren, top) and in- frared (2MASS and WISE, bottom) photometric systems Syn- to the infrared collected from literature. To eliminate the 3 thetic SEDs are plotted as Fλλ . In the top panel a single SED steep slope of the SED we plot the flux density times the is plotted, while for the bottom panel a composite SED is used. 3 wavelength to the power of three (Fλλ ) as a function of Both are scaled for comparison to the transmission functions. wavelength throughout this paper. 2.2 Ultraviolet fluxes from IUE spectra 2.1 Photometric data The IUE satellite provided UV spectra for two wave- length ranges; the short (SW, 1150-1975Å) and long The visual range is covered by SDSS (Alam et al. 2015) (1910-3150Å) wavelength range. Each spectrograph of- and APASS (Henden et al. 2016) data as well as mag- fered both high and low resolution modes, with spectral nitudes and colours in the Johnson-Cousins, Strömgren resolutions of 0.2 and 6 Å respectively, as well as two en- (see Fig. 1), and Geneva systems, which are collected us- trance apertures each, a small circular aperture with a ing Vizier as well as the Subdwarf Database1 by Østensen 3 arcsec diameter and a large rectangular aperture of 10 (2006). by 20 arcsecs. We discarded spectra at high-resolution as Ultraviolet fluxes are important to constrain the at- well as those taken through a small aperture, because the mospheric parameters of a hot subdwarf and were ex- flux calibration is less accurate than that for the large- tracted from observations by the International Ultraviolet aperture, low-resolution spectra. Because we have to com- Explorer (IUE), available in the MAST2 archive. bine them with broad and intermediate band optical and Infrared photometry is of particular importance for infrared photometry we defined a suitable set of filters binary systems that contain a cool companion because an to derive UV-magnitudes from IUE spectra (see Fig. 2). IR excess is expected. Available infrared data were taken Three box filters, which cover the spectral ranges 1300– from ALLWISE (Wright et al. 2010; Cutri & et al. 2013), 1800 Å, 2000–2500 Å, and 2500–3000 Å, are defined to ex- 2MASS (Skrutskie et al. 2006), and UKIDSS (Lawrence tract magnitudes from the IUE spectra. The box filters et al. 2007, see Fig. 1). were designed in order to avoid the boundaries of the The photometric datasets are inhomogeneous, both SW and LW wavelength ranges because of the increasing with respect to bandwidth as well as to accuracy. Ul- noise level and the region around the Lyman-alpha line traviolet spectra from IUE cover the wavelength range because of the contribution by interstellar gas absorption. from 1150Å to 3150Å at a spectral resolution of 6 Å. The The mid-UV filter was designed to include the UV ab- optical spectral range is covered by several filters both sorption bump at ≈ 2200Å of interstellar absorption (see narrow band (e.g. Strömgren) and wide-band (e.g. Sloan Fig. 3), which is important to determine the interstellar or Johnson), while the infrared is usually represented by reddening parameter E(B-V). five wide-band filters (J, H, K, W1, and W2). 1 http://catserver.ing.iac.es/sddb/ 2 http://archive.stsci.edu/ SEDs and colours of sdB stars 3 0.004 1 0.003 unreddened 0.8 Extinction factor 3 box λ E(B-V)=0.15 · 0.002 reddening law f 0.6 0.001 (arbitrary units) 0.4 3 λ 0 · λ 1200 1400 1600 1800 2000 F 0.2 Wavelength [A]˚ 0.004 0 1000 2000 5000 10000 20000 50000 box 0.003 box Wavelength [A]˚ 3 Fig. 3. The impact of interstellar reddening on SEDs of a B star λ · 0.002 for moderate interstellar reddening by E(B-V)=0.15 mag. The f reddening law is from Fitzpatrick (1999). 0.001 extinction in magnitude at wavelength λ, A(λ), as a func- 0 2000 2500 3000 tion of the colour excess E(B −V ) and the extinction pa- Wavelength [A]˚ rameter RV = A(V )/E(B−V ) (defaulted to 3.1) is taken from Fitzpatrick (1999, see Fig. 3). The final expression Fig. 2. Box filters to convert IUE low resolution spectra to UV to calculate a synthetic magnitude, therefore, reads as magnitudes. One box is placed in the wavelength range covered ∞ 2 R −0.4A(λ) by the short wavelength camera (SWP, upper panel) and two in Θ rx(λ)10 F (λ)λdλ the regime covered by the long wavelength cameras (LWR/LWP, 0 ref magx = −2.5 log ∞ +magx . lower panel). R ref 4 rx(λ)f (λ)λdλ 0 (2) 3 Synthetic SEDs and colours The magnitude magx of an arbitrary photometric pass- 3.1 Grids of synthetic SEDs and colours band x is defined as ∞ We aim at modelling the observed SEDs and colours of R rx(λ)f(λ)λdλ single sdB stars or SB1 binaries, as well as composite 0 ref magx = −2.5 log ∞ + magx (1) spectrum systems consisting of a hot subdwarf and a late- R ref rx(λ)f (λ)λdλ type main-sequence star.
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