Hot subdwarf stars from the Hamburg Quasar Survey

Abstract

Hot subluminous stars (sdO/B) are evolved low mass stars originating from red giants that lost their envelope almost entirely. The multitude of observed phenomena imply that several pathways may form hot subdwarfs, most involving close binary channels. The Hamburg Quasar Survey (HQS) led to the discovery of many faint blue stars including hot subdwarf. Many of the HQS-sdB stars have been studied in detail, but analyses of the helium-rich sdOB and sdO stars are lacking. The recent development of hybrid LTE/non-LTE model spectra 2nd generation Bamberg model grids enables us to improve the spectroscopic analyses of the sdB stars as well as of the previously unstudied sdO stars allowing precise atmospheric parameters to be derived, while consistently accounting for parameter correlations and systematic uncertainties. ... We use spectral energy distributions to identify composite-colour sdB binaries and present the result of detailed spectroscopic analyses of 122 non-composite subdwarfs from the HQS to identify potential evolutionary pathways. ...Their derived mass distribution and median mass of 0.45 Msun is consistent with the canonical EHB mass. ... The helium-rich sdOB and sdO stars, are found near the helium main-sequence (He-MS). The derived mass distribution of the extremely He-rich subdwarfs is broader (0.48 to 1.05 Msun) and peaks at a median of 0.70 Msun, significantly larger than those of the hydrogen-rich stars. Intermediate He-rich subdwarfs are also He-MS stars, but of lower mass (0.55 Msun) than the extremely He-rich. This strongly supports the merger scenario for the origin of He-rich sdO stars, in which two helium white dwarfs merge following orbital decay driven by gravitational-wave emission, producing a He-rich sdO or sdOB star. From comparison to the results of similar studies we speculate that older populations produce more massive He-WD mergers.(abbreviated)

Figures (15)

• Figure 1: Top: Distribution of the non-composite HQS subdwarfs in Galactic coordinates, colour coded with the monochromatic interstellar reddening parameter $E(44-55)$ (see Sect. \ref{['sect:ism']}); bottom: a histogram of distances from the Galactic plane $z$ for all stars with parallax errors better than 25% (bottom) calculated from their Gaia parallaxes (see Sect. \ref{['sect:astrometry']} for details).
• Figure 2: Example spectral fits (red) to DSAZ spectra (black) of the main types of hot subdwarfs in HQS.
• Figure 3: Distribution of parallax uncertainties as a function of parallax. The dashed dotted, and the dashed lines mark the 10% and 25% uncertainty levels, respectively. HS0941+4649 and HS1000+4704 are off the scale; their parallax uncertainties are larger than the parallax. He-poor stars are shown in blue, iHe rich ones in black and eHe-rich in red.
• Figure 4: Dimension of the model atmosphere grid in the $T_{\text{eff}}$-$\log g$ plane (black line). The grey band denotes the EHB for solar composition Dorman1993 for comparison. The helium main sequence 1971AcA....21....1P is shown in red with masses labelled. The dashed line shows the Eddington limit for solar composition 1988ApJ...324..279L. Grids are constructed at metal composition of 1/100, 1/10, 1 and 10 times the standard sdB metallicity pattern and $n$(He)/$n$(H) of 1/10,000 to $\approx$ 100 times solar, depending on metal content.
• Figure 5: Correlation of $T_{\text{eff}}$ with $\log g$: correlation factors as a function of $T_{\text{eff}}$ for He-poor (blue), intermediate helium-rich (black), and extremely He-rich subdwarfs (red).