111 research outputs found
The Hot R Coronae Borealis Star DY Centauri is a Binary
The remarkable hot R Coronae Borealis (RCB) star DY Cen is revealed to be the first and only binary system to be found among the RCB stars and their likely relatives, including the extreme helium stars and the hydrogen-deficient carbon stars. Radial velocity determinations from 1982 to 2010 have shown that DY Cen is a single-lined spectroscopic binary in an eccentric orbit with a period of 39.67 days. It is also one of the hottest and most H-rich member of the class of RCB stars. The system may have evolved from a common envelope to its current form.Spanish Ministry of Economy and Competitiveness AYA-2011-27754McDonald Observator
The M Dwarf Problem in the Galaxy
We present evidence that there is an M dwarf problem similar to the
previously identified G dwarf and K dwarf problems: the number of
low-metallicity M dwarfs is not sufficient to match simple closed-box models of
local Galactic chemical evolution. We estimated the metallicity of 4141 M dwarf
stars with spectra from the Sloan Digital Sky Survey (SDSS) using a molecular
band strength versus metallicity calibration developed using high resolution
spectra of nearby M dwarfs. Using a sample of M dwarfs with measured
magnitudes, parallaxes, and metallicities, we derived a relation that describes
the absolute magnitude variation as a function of metallicity. When we examined
the metallicity distribution of SDSS stars, after correcting for the different
volumes sampled by the magnitude-limited survey, we found that there is an M
dwarf problem, with the number of M dwarfs at [Fe/H] ~ -0.5 less than 1% the
number at [Fe/H] = 0, where a simple model of Galactic chemical evolution
predicts a more gradual drop in star numbers with decreasing metallicity.Comment: To be published in Monthly Notices of the RAS by the Royal
Astronomical Society and Blackwell Publishing. 7 pages, 3 figure
Subaru and salt spectroscopy of chemically peculiar hot subdwarfs
The majority of hot subdwarfs lie on or close to the helium main-sequence. Many have hydrogen-rich surfaces, but a substantial fraction of the hotter subdwarfs have hydrogen-depleted or hydrogen-deficient surfaces. Amongst the former, three were known to show extraordinary overabundances of heavy elements including zirconium and lead. Using Subaru/HDS,we commenced a high-resolution survey of hydrogen-depleted subdwarfs to discover new members of the class. UVO 0825+15, was found to exhibit strong lead lines, to be an intrinsic variable in K2 field 5, and to have a relatively high space motion. Two other lead-rich subdwarfs have been found in the Subaru sample. A much wider survey is in progress using SALT/HRS. Discoveries so far include one extreme helium star similar to V652 Her, and an intermediate helium star with possible comparison to HD144941. Analyses of the hotter and more compact members of the sample are continuing. Open Access
Subaru and SALT spectroscopy of chemically peculiar hot subdwarfs
The majority of hot subdwarfs lie on or close to the helium main-sequence. Many have hydrogen-rich surfaces, but a substantial fraction of the hotter subdwarfs have hydrogen-depleted or hydrogen-deficient surfaces. Amongst the former, three were known to show extraordinary overabundances of heavy elements including zirconium and lead. Using Subaru/HDS,we commenced a high-resolution survey of hydrogen-depleted subdwarfs to discover new members of the class. UVO 0825+15, was found to exhibit strong lead lines, to be an intrinsic variable in K2 field 5, and to have a relatively high space motion. Two other lead-rich subdwarfs have been found in the Subaru sample. A much wider survey is in progress using SALT/HRS. Discoveries so far include one extreme helium star similar to V652 Her, and an intermediate helium star with possible comparison to HD144941. Analyses of the hotter and more compact members of the sample are continuing
Metallicity measurements using atomic lines in M and K dwarf stars
We report the first survey of chemical abundances in M and K dwarf stars
using atomic absorption lines in high resolution spectra. We have measured Fe
and Ti abundances in 35 M and K dwarf stars using equivalent widths measured
from (lambda / Delta lambda) = 33,000 spectra. Our analysis takes advantage of
recent improvements in model atmospheres of low-temperature dwarf stars. The
stars have temperatures between 3300 and 4700 K, with most cooler than 4100 K.
They cover an iron abundance range of -2.44 < [Fe/H] < +0.16. Our measurements
show [Ti/Fe] decreasing with increasing [Fe/H], a trend similar to that
measured for warmer stars where abundance analysis techniques have been tested
more thoroughly. This study is a step toward the observational calibration of
procedures to estimate the metallicity of low-mass dwarf stars using
photometric and low-resolution spectral indices.Comment: 6 pages, LaTeX. To appear in MNRAS. Full version of Table 2 available
at http://www.astro.washington.edu/vmw/mnras/table2.pd
Radial velocity variations of the pulsating subdwarf B star PG 1605+072
We present an analysis of high-speed spectroscopy of the pulsating subdwarf B
star PG 1605+072. Periodic radial motions are detected at frequencies similar
to those reported for photometric variations in the star, with amplitudes of up
to 6 km/s. Differences between relative strengths for given frequency peaks for
our velocity data and previously measured photometry are probably a result of
shifting of power between modes over time. Small differences in the detected
frequencies may also indicate mode-shifting. We report the detection of
line-shape variations using the moments of the cross correlation function
profiles. It may be possible to use the moments to identify the star's
pulsation modes.Comment: 5 pages, 6 figure
The 1995-1996 Decline of R Coronae Borealis - High Resolution Optical Spectroscopy
A set of high-resolution optical spectra of RCrB acquired before, during, and
after its 1995-1996 decline is discussed. All of the components reported from
earlier declines are seen. This novel dataset provides new information on these
components including several aspects not previously seen in declines of RCrB
and other RCBs. In the latter category is the discovery that the decline's
onset is marked by distortions of absorption lines of high-excitation lines,
and quickly followed by emission in these and in low excitation lines. This
'photospheric trigger' implies that dust causing the decline is formed close to
the star. These emission lines fade quickly. After 1995 November 2, low
excitation narrow (FWHM ~12 km s-1) emission lines remain. These appear to be a
permanent feature, slightly blue-shifted from the systemic velocity, and
unaffected by the decline except for a late and slight decrease of flux at
minimum light. The location of the warm, dense gas providing these lines is
uncertain. Absorption lines unaffected by overlying sharp emission are greatly
broadened, weakened, and red-shifted at the faintest magnitudes when scattered
light from the star is a greater contributor than direct light transmitted
through the fresh soot cloud. A few broad lines are seen at and near minimum
light with approxiamately constant flux: prominent among these are the He I
triplet series, Na I D, and [N II] lines. These lines are blue-shifted by about
30 km s(-1) relative to the systemic velocity with no change in velocity over
the several months for whicht he lines were seen. It is suggested that these
lines, especially the He I lines, arise from an accretion disk around an unseen
compact companion, which may be a low-mass white dwarf. If so, R CrB is similar
to the unusual post-AGB star 89 Her.Comment: 31 pages, 26 figure
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