SSG Variable Star Research
Close Binary M Dwarf-White Dwarf Systems
Pre-CVs and the missing magnetic Pre-CV problem.
White Dwarf Star Ages and Kinematics
This project, primarily carried out by SSG member Nicole Silvestri, her UW collaborators, and her undergraduate students, involves the study of the evolution of dMs in close WD+dM binary systems, often referred to as pre-cataclysmic variables (pre-CVs), drawn from the Sloan Digital Sky Survey (SDSS). The ultimate goal of this project is to fully understand how the close binary environment influences the evolution of the low mass secondary and to compare these results to a variety of recent low mass star studies in the literature (Reid et al. 1995, Hawley et al. 1996, West et al. 2004, to name a few) and to the evolution of low mass stars in wide binaries as discussed below.
As the progenitors to CVs, many of which eventually evolve to become Type 1a supernovae (SNe), pre-CVs are integral in understanding some of the more vexing questions associated with the current observed CV population, such as magnetic CV progenitors and the "period gap". The prolific use of Type 1a SNe for groundbreaking cosmology has also lead to an increased interest in the pre-CV environment as we have yet to fully understand the origin and nature of Type 1a progenitors. A detailed study of pre-CV systems allows us to test widely used theories on common envelope (CE) evolution and orbital angular momentum loss (magnetic breaking). Most of what we know about these theories are generally inferred from the properties of the current CV population, which is complicated by the significant effects of mass transfer.
Our current sample of pre-CVs consists of over 1600 spectroscopically identified pairs discovered in the SDSS (see Figure 1). Raymond et al. (2003) laid the groundwork for this project and showed that to properly analyze the properties of both components of the pre-CV, great care must be taken to effectively separate the WD flux from the dM flux in the combined SDSS spectrum. I have improved upon this procedure with an iterative method involving the subsequent fitting and subtraction of best fit SDSS template dM spectra (Hawley et al. 2002) and model WD spectra of D. Koester (see Kleinman et al. 2004) from the combined DA+dM SDSS spectra in our sample (see Silvestri et al. 2005b).
This is the most surprising outcome of the projects carried out by SSG member Nicole Silvestri, her UW collaborators, and her undergraduate students. While assessing our growing number of WD+dM pre-CV pairs we have discovered that very few of the WDs in these pairs appear to be magnetic (see Lemagie et al. 2005, Silvestri et al. 2005b). Less than 10 out of over 600 DA+dM pre-CVs fitted with magnetic hydrogen WD models were determined to have a magnetic field, and in all cases the field was small (< 10 MG). It is intriguing that nearly 25% of all known CV systems harbor a magnetic WD with an appreciable magnetic field (~30-50 MG) and yet we see very few pre-CVs with magnetic WDs.
SDSS J121209.31+013627.7, discovered by Schmidt et al. (2003) may be the first magnetic WD found to have a non-degenerate companion. We have shown that the magnetic WD has a significant amount of emission in both Halpha and Hbeta (Figure 4a,b) and a large velocity variation indicating the existence of a close companion. Magnetic WDs have been shown to be more massive than their non-magnetic counterparts (Liebert, Bergeron, & Holberg 2005), therefore we believe the companion is most likely a low mass late M dwarf or a brown dwarf. I was granted three half nights on the Astrophysical Research Consortium's 3.5-m telescope for radial velocity followup to determine the orbital period.
This project, primarily carried out by SSG member Nicole Silvestri, her UW collaborators, and her undergraduate students uses the wide binaries from her thesis comprised of a white dwarf (WD) star and dM (WD+dM) with average separations of ~100's AU. The binaries are coeval, therefore age of the WD, as determined from WD evolutionary models (Bergeron et al. 1995), is the age of the dM companion. Using a sample of approximately 200 WD+dM CPMBs we were able to confirm that dMs remain active for a longer time at later spectral type, a trend established in studies of dMs in clusters. We were also successful at extending these relations to ages much older than can be examined using dMs in open clusters. Our study revealed a tremendous amount of complexity in assigning age based on activity alone (Silvestri et al. 2005a).
We also investigate the kinematics and space motion of the CPMB populations and used this sample to address the dark matter content of the halo. We showed that the use of kinematics as the sole means for identifying potential halo members can lead to erroneous estimates of the number of halo WD. Using the variety of temperature and metallicity sensitive features present in dM spectra, we demonstrated that even though a large fraction of our CPMBs have halo-like kinematics, only one of the dMs had a metallicity indicative of the halo population (Silvestri et al. 2001). The rest are likely members of the high velocity tail of the thick disk component of the Galaxy.
M Dwarf Science
Chromospheric Activity and Ages of M Dwarf stars
Common Proper Motion Binary M Dwarf-White Dwarf Systems
This project, primarily carried out by SSG member Nicole Silvestri, her UW collaborators, and her undergraduate students aims to obtain the effective temperatures and log(g) estimates of the separated WD spectra from the atmospheric models of D. Koester and rough ages from WD evolutionary models (Bergeron et al. 1995). We use the TiO bandhead ratios of the dMs to determine the spectral type (mass, Hawley et al. 1996), activity level (Halpha EW and L_Halpha/L_bol), and distance. We measure the radial velocities and full space motions (U, V, W) of each system to determine the population membership of the sample. We have approached the study of these systems from several avenues, the full results of which was presented at the 2005 SDSS Special Session of the January AAS meeting in San Diego and in (Lemagie et al. 2005, Silvestri et al. 2005b) and in two publications (Silvestri et al. 2005c,d). We have found some interesting and surprising results. I highlight a few here:
The current sample of pre-CV secondaries shows enhanced activity as compared to a large sample of field dMs in the SDSS. This is interesting, though not terribly surprising, given the potential for the close binary environment to increase the spin of the secondary due to tidal interaction, thereby increasing the activity. Nearly 31% of early dMs and 84% of mid-dMs are active in this pre-CV sample as compared to 2% and 34% in West et al. (2004), respectively. Irradiation of the secondary by the hot WD companion is also a likely contributer to the observed activity in these systems.
The orbital periods determined for a small subset of our sample show a large range in periods. The longest periods are 10's of hours. The shortest are very close to the onset of mass transfer (~4 hours) and a few periods fall within the CV "period gap" (2 < P_orb < 3 hours). This large range from long to short orbital periods can be taken as snapshots in time of the orbital evolution of a given pre-CV system - starting with the pair's emergence from the CE through the onset of mass transfer, providing a unique look at the evolution of a pre-CV system over time.
We use common proper motion binary (CPMB) systems first identified by W. Luyten (1963) to study activity in low mass stars. The goal was to develop a chromospheric activity-age relation for low mass M dwarfs (dMs; see Silvestri et al. 2001; Silvestri 2002; Silvestri et al. 2002, 2005a) similar to the one for F, G,and K dwarf stars (Skumanich 1972), though due in part to structural differences in the M dwarf sequence is significantly more complex than the current relation (F(Ca K) = At^-1/2). We aimed to extend these relations to ages much larger than those determined by studies of dMs in open clusters (see Reid et al. 1995, Hawley et al. 1996, and references therein).
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