The Origins of Hot Subdwarf Stars as Illuminated by Composite-Spectrum Binaries

I. Introduction

Hot subdwarf stars are a class of stars defined as having temperatures exceeding 20,000 K, with surface gravities higher and luminosities lower than normal hydrogen-burning main sequence stars of the same temperature. There are two major types of hot subdwarfs defined based on the appearance of their spectrum: subdwarf B (sdB; dominated by hydrogen lines) and subdwarf O (sdO; dominated by helium lines). I have been focusing on the more numerous sdB stars. This apparently homogeneous population of objects with similar formation histories forms a unique laboratory for the study of post-main sequence stellar evolution and binary evolution.

In a plot of luminosity vs. temperature (called a Hertzsprung-Russell Diagram, or just HR Diagram), hot subdwarfs fall into the hottest part of the region occupied by evolved stars which are burning helium in their cores. All helium burning stars lie on what is called the ``horizontal branch'' because they all have basically the same luminosity, but a wide range in temperatures (they form a nearly horizontal line across the HR Diagram). Thus the field sdB stars are understood as stars with a total mass of about half the mass of the Sun, which have only a thin skin of hydrogen (approximately 1/100-th of the mass of the Sun) surrounding a helium burning core1.

The leading theory describing the formation of sdB stars in our Galaxy claims that they lost significant mass (up to the total mass of our Sun) due to interactions with a binary companion sometime during their post-main sequence evolution2. Thus finding and understanding the companions to sdB stars will help place constraints on many aspects of binary evolution and stellar mass loss.

II. Composite Spectrum sdB Stars

It has been observed that some sdB stars show features in their spectrum that cannot be attributed to the sdB star alone. These ``composite spectrum'' sdB stars are in fact a blend of two unresolved stars of very different temperatures: the hot sdB (Temperature > 20,000 K) and a cooler star (~6000 K > T > ~4000 K). The exact nature of these cooler stars is currently disputed. Some believe they are normal hydrogen-burning main sequence stars3, others believe they are more evolved (and thus more luminous) sub-giant or giant stars4. I am working to remove some of the assumptions typically used when studying these composite systems, by actually determining the properties of the cool companions directly.

I conducted a photometric, large-scale search for sdB+cool composite systems utilizing the Two Micron All Sky Survey (2MASS). 2MASS is a near-infrared (near-IR) imaging survey preformed in three wavelength regions: ~1.2-1.3 mu-m, ~1.5-1.8 mu-m, and ~2.0-2.3 mu-m [1 mu-m (micro-meter) = 0.000001 meters (or one-millionth of a meter); for comparison, visible light is ~0.4-0.8 mu-m]. Since sdB stars and their cool companions have drastically different temperatures, they have different effects on the blended energy output at different wavelengths: the sdB star dominates in the ultraviolet, while the cool companion dominates in the near-IR. Thus, using 2MASS I was able to identify sdB stars that showed near-IR colors which indicated the presence of a cool companion5. I found that ~40% of known sdB stars show evidence for the presence of an unresolved cool companion, however, applying corrections to account for selection and sensitivity biases this fraction drops to ~25%5.

III. The Nature of Late-Type Companions

The majority of my thesis focuses on a sub-set of the 2MASS sample of composite sdB+cool companion systems. For this sample I obtained optical spectra using the McDonald 2.7m telescope with the Large Cassegrain Spectrograph (LCS) and the Kitt Peak National Observatory (KPNO) 2.1m telescope with the GoldCam Spectrograph. I chose to concentrate on the ``red'' portion of the spectrum where the features of the cool companion will be least diluted by the sdB star. I am combining this spectroscopy with the near-IR photometry from 2MASS and visual photometry from a variety of sources in order to provide additional constraints on the properties of the stars. Figure 1 shows three sample spectra taken with the KPNO-GoldCam: a 2MASS identified single sdB, a 2MASS identified composite sdB, and a ``standard star'' which is a cool (T ~ 5200 K) main sequence star. The sdB star dominates the spectrum at the short wavelength end (compare the features in the left end of the composite spectrum with the single sdB), and the cool companion dominates at the long wavelength end (compare the features in the right end of the composite spectrum with the cool standard star).

By comparing the composite sdB spectra with models based on ``standard stars'' with a variety of temperatures and either main sequence or sub-giant, I found that, in most cases, the cool stars can be accounted for by main sequence companions6. Models with subgiants are less successful, but they are not excluded in some cases. Further analysis of additional spectral features and refinement of the model grids is needed to distinguish between main sequence and subgiant in these cases.

Uncovering the nature of these unresolved companions will place constraints on binary formation theories and mechanisms, as well as post-main sequence evolution and mass loss. Understanding the cool companions will also refine our knowledge of the sdB star's physical properties (i.e., luminosity, mass, size, and distance).


Figure 1. Example spectra for a single sdB (top), a composite sdB (middle), and a cool standard (bottom) offset by constants. It is evident that the cool companion dominates at longer wavelengths. SdB features from hydrogen (H-alpha & H-beta) and helium (He I), along with cool star features from magnesium (Mg I), sodium (Na I), and calcium (Ca II) are marked. (Location in text)

References

  1. Dorman, Rood, & O'Connell, 1993, ApJ, 419, 596; Brassard, Fontaine, Billères, Charpinet, Liebert, & Saffer, 2001, ApJ, 563, 1013; in text
  2. Mengel, Norris, & Gross, 1976, ApJ, 204, 488; Han, Podsiadlowski, Maxted, Marsh, & Ivanova, 2002, MNRAS, 336, 449; Han, Podsiadlowski, Maxted, & Marsh, 2003, MNRAS, 341, 669; in text
  3. i.e., Allard, Wesemael, Fontaine, Bergeron, & Lamontagne, 1994, AJ, 107, 1565; Ferguson, Green, & Liebert, 1984, ApJ, 287, 320; Bixler, Bowyer, & Laget, 1991, A&A, 250, 370; in text
  4. i.e., Jeffery, & Pollacco, 1998, MNRAS, 298, 179; Theissen, Moehler, Heber, Schmidt, & de Boer, 1995, A&A, 298, 577; in text
  5. Stark, & Wade, 2003, AJ, 126, 1455; Stark, Wade, & Berriman, 2004, Ap&SS, 291, 333; in text
  6. Stark & Wade, 2004, in ASP Conf. Ser.: 14th European Workshop on White Dwarfs, submitted (astro-ph/0410286); in text

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