Revealing Unresolved White Dwarf-Main Sequence Binaries using Gaia DR3 and GALEX I. A Volume limited study of 100 pc

Abstract

Context. Understanding the demographics of white dwarf - main sequence (WDMS) binaries is key to uncovering the formation of various stellar exotica and refining the details of binary stellar evolution. Despite several dedicated efforts to identify unresolved WDMS binaries, their population remains incomplete, even within a 100 pc volume-limited sample. Aims. This study aims to identify WDMS binaries hidden within the main sequence of the optical color-magnitude diagram (CMD), improving the completeness of WDMS binaries within a volume-limited sample of 100 pc. Methods. We use NUV-optical CMDs to distinguish unresolved WDMS binaries from the rest of the populations. High-precision astrometric and photometric data from Gaia DR3 and NUV data from GALEX GR6/7 are combined to construct CMDs. Using the binary spectral energy distribution (SED) fitting algorithm within the Virtual Observatory SED Analyzer (VOSA) tool, we estimate stellar parameters such as effective temperature, bolometric luminosity, and radii. The masses of the WD companions are determined using white dwarf evolutionary models. As we use the sources which are detected only in NUV band of GALEX, this study directly complements to majority of the previous studies. Results. We identify 596 WD-MS binary candidates within 100 pc, with 497 newly reported. Our method predominantly identifies binaries with cooler WD companions (median ~7,000 K) compared to previous studies. The WD masses range from ~0.2 and 1.3 M$_\odot$, and most MS companions are of M spectral type.

Figures (11)

• Figure 1: The CMDs for sources within 100 pc in the absolute plane after correcting for distance modulus and extinction, where the subscript "abs" stands for absolute magnitude. In both the panel, isochrones of 100 Myr (cyan line) and 1 Gyr (black line) are overlaid to indicate the evolutionary sequences, and the grey asterisks indicate main-sequence (MS) and post-MS populations. Left: UV-Optical CMD (NUV$-$BP vs NUV). The red line separates the hotter and NUV-bright sources (blue asterisks, green circles and orange plus) from the others (grey asterisks) on the NUV-optical CMD. Right: Optical CMD (BP$-$RP vs G). The red line separates the blue asterisks (WD populations) from the rest of the NUV-bright sources (green circles and orange plus) which are located on the MS or with within the gap region between WD and MS. These green circles represent WDMS binary candidates. Orange plus highlights the candidates for which we could fit observed SED from UV to IR with WDMS composite model fluxes using the VOSA tool.
• Figure 2: Two examples of well-fitted SEDs of WDMS binaries. Index number as per \ref{['tab:source_table']}, Gaia DR3 source ID, the values of A$_V$, ${ \chi^2_{\rm{red}} }$, Vgf and Vgf$_b$ are mentioned on top of each panel. (Top panel:) Cyan points (with blue errors) denote the observed flux from UV to IR. The observed data points with upper-limit on flux or unreliable detections are marked as asterisks, and they are not included in the fit. The black (grey) line represents the best-fit synthetic spectra of WD (MS). The red points indicate the expected combined model fluxes from the best-fit synthetic spectra. (Bottom panel:) Fractional residue fluxes are shown in different bands. The blue and red dashed lines represent 50% and 100% residue flux, while the black line represents zero residue. There is excellent agreement between the cyan and red points, and less than 50% residue fluxes in every band indicating a well-fitted SED.
• Figure 3: Same as \ref{['sed']} but for two representative examples of badly-fitted SEDs of WDMS candidate binaries. Index number as per the online catalog at the CDS, Gaia DR3 source ID, the values of A$_V$, ${ \chi^2_{\rm{red}} }$, and Vgf$_b$ are mentioned at the top. In the case of source 415 (left), the model spectra are unable to fit the observed flux for majority of data points despite their residual flux $\le$50% and Vgf$_b$ value $<$ 15. For source 416 (right), though the model spectra nicely fit the observed fluxes in the UV and optical region but only a couple of observed data points are fitted with MS spectrum due to lack of data points in the NIR regions, hence the fit is not reliable.
• Figure 4: Distribution of ${T_{\rm{eff}}}$ for the WDs (blue) and MS (orange) stars in our candidate WDMS binaries. The median, 25th, and 75th percentiles for the MS s (WD) is ${T_{\rm{eff}}}/\rm{K}=3100^{+100}_{-200}$ ($6250^{+2000}_{-750}$).
• Figure 5: Same as \ref{['teff']}, but showing the stellar radii for WDs (blue) and MS stars (orange). The median, 25th, and 75th percentiles for the MS stars (WDs) is $\log(R/R_\odot)=-0.40^{+0.14}_{-0.18}$ ($-1.82^{+0.39}_{-0.28}$).