16 research outputs found
Forged by giants: understanding the dwarf carbon stars
Dwarf carbon (dC) stars are main-sequence stars with carbon molecular bands (C_2, CN, CH) in their optical spectra. They are an important class of post-mass transfer binaries since, as main-sequence stars, dCs cannot have produced carbon themselves. Rather, the excess carbon originated in an evolved companion, now a white dwarf, and was transferred to the dC. Because of their complex histories, dCs are an excellent sample for testing stellar physics, including common-envelope evolution, wind accretion, mass transfer efficiencies, and accretion spin-up. However, their fundamental properties remain a mystery, and this impedes efforts to use dCs to constrain the evolution of binary systems.
Here, I have investigated the observed properties of dCs, both as a population and as individual objects. Using multi-epoch spectroscopy, I constrained the dC binary fraction to be consistent with 100% binarity. The best-fit orbital separation distribution agrees with the few known dC orbital periods, and suggests a bimodal distribution (one sample with mean periods of hundreds of days, the other thousands of days). I also built a set of optical templates to find and classify additional dCs in spectroscopic surveys.
Further, I discovered periodic variability in photometry of 34 dCs, dramatically increasing the number of measured periods. This allowed me to investigate mass transfer mechanisms that are likely to be important in the formation of dCs. Interestingly, some of these objects have short periods (P < 2d), indicating they have gone through a common-envelope phase. I explored the implications of these short-period dCs and how they will allow for constraints to be placed on the physics of common-envelope evolution.
Finally, I searched for signs of spin-up and activity in dCs using X-ray emission. From this, I found that dCs are consistent with being rapid rotators, similar to what is observed in samples of normal young dwarfs.
In summary, this dissertation presents the most extensive set of dC observational properties that has been compiled to date. I have confirmed the binary origin of dCs and linked some to post-common-envelope binaries. My work has provided a firmer foundation for the use of dCs to explore many essential astrophysical phenomena
Classifying Single Stars and Spectroscopic Binaries Using Optical Stellar Templates
Stellar spectral classification is a fundamental tool of modern astronomy,
providing insight into physical characteristics such as effective temperature,
surface gravity, and metallicity. Accurate and fast spectral typing is an
integral need for large all-sky spectroscopic surveys like the SDSS and LAMOST.
Here, we present the next version of PyHammer, stellar spectral classification
software that uses optical spectral templates and spectral line index
measurements. PyHammer v2.0 extends the classification power to include carbon
(C) stars, DA white dwarf (WD) stars, and also double-lined spectroscopic
binaries (SB2). This release also includes a new empirical library of
luminosity-normalized spectra that can be used to flux calibrate observed
spectra, or to create synthetic SB2 spectra. We have generated physically
reasonable SB2 combinations as templates, adding to PyHammer the ability to
spectrally type SB2s. We test classification success rates on SB2 spectra,
generated from the SDSS, across a wide range of spectral types and
signal-to-noise ratios. Within the defined range of pairings described, more
than of SB2s are correctly classified.Comment: 16 pages, 7 figures, 4 tables; accepted to ApJ
A Chandra Study: Are Dwarf Carbon Stars Spun Up and Rejuvenated by Mass Transfer?
Carbon stars (with C/O> 1) were long assumed to all be giants, because only
AGB stars dredge up significant carbon into their atmospheres. The case is
nearly iron-clad now that the formerly mysterious dwarf carbon (dC) stars are
actually far more common than C giants, and have accreted carbon-rich material
from a former AGB companion, yielding a white dwarf and a dC star that has
gained both significant mass and angular momentum. Some such dC systems have
undergone a planetary nebula phase, and some may evolve to become CH, CEMP, or
Ba giants. Recent studies indicate that most dCs are likely from older,
metal-poor kinematic populations. Given the well-known anti-correlation of age
and activity, dCs would not be expected to show significant X-ray emission
related to coronal activity. However, accretion spin-up might be expected to
rejuvenate magnetic dynamos in these post mass-transfer binary systems. We
describe our Chandra pilot study of six dCs selected from the SDSS for Halpha
emission and/or a hot white dwarf companion, to test whether their X-ray
emission strength and spectral properties are consistent with a rejuvenated
dynamo. We detect all 6 dCs in the sample, which have X-ray luminosities
ranging from logLx= 28.5 - 29.7, preliminary evidence that dCs may be active at
a level consistent with stars that have short rotation periods of several days
or less. More definitive results require a sample of typical dCs with deeper
X-ray observations to better constrain their plasma temperatures.Comment: 13 pages, 5 figures. Revised and resubmitted June 20, accepted June
21, 2019 to Ap
The fastest stars in the Galaxy
We report a spectroscopic search for hypervelocity white dwarfs (WDs) that
are runaways from Type Ia supernovae (SNe Ia) and related thermonuclear
explosions. Candidates are selected from Gaia data with high tangential
velocities and blue colors. We find six new runaways, including four stars with
radial velocities (RVs) and total space velocities
. These are most likely the surviving donors from
double-degenerate binaries in which the other WD exploded. The other two
objects have lower minimum velocities, , and may
have formed through a different mechanism, such as pure deflagration of a WD in
a Type Iax supernova. The four fastest stars are hotter and smaller than the
previously known "D stars," with effective temperatures ranging from
20,000 to 130,000 K and radii of . Three
of these have carbon-dominated atmospheres, and one has a helium-dominated
atmosphere. Two stars have RVs of and -- the
fastest systemic stellar RVs ever measured. Their inferred birth velocities,
, imply that both WDs in the progenitor binary
had masses . The high observed velocities suggest that a
dominant fraction of the observed hypervelocity WD population comes from
double-degenerate binaries whose total mass significantly exceeds the
Chandrasekhar limit. However, the two nearest and faintest D stars have the
lowest velocities and masses, suggesting that observational selection effects
favor rarer, higher-mass stars. A significant population of fainter low-mass
runaways may still await discovery. We infer a birth rate of D stars that
is consistent with the SN Ia rate. The birth rate is poorly constrained,
however, because the luminosities and lifetimes of stars are
uncertain.Comment: 26 pages, 17 figures. Accepted to OJ