36 research outputs found
Towards an Understanding of the Atmospheres of Cool White Dwarfs
Cool white dwarfs with Teff < 6000 K are the remnants of the oldest stars
that existed in our Galaxy. Their atmospheres, when properly characterized, can
provide valuable information on white dwarf evolution and ultimately star
formation through the history of the Milky Way. Understanding the atmospheres
of these stars requires joined observational effort and reliable atmosphere
modeling. We discuss and analyze recent observations of the near-ultraviolet
(UV) and near-infrared (IR) spectrum of several cool white dwarfs including
DQ/DQp stars showing carbon in their spectra. We present fits to the entire
spectral energy distribution (SED) of selected cool stars, showing that the
current pure-hydrogen atmosphere models are quite reliable, especially in the
near-UV spectral region. Recently, we also performed an analysis of the coolest
known DQ/DQp stars investigating further the origin of the C2 Swan bands-like
spectral features that characterize the DQp stars. We show that the carbon
abundances derived for DQp stars fit the trend of carbon abundance with Teff
seen in normal cool DQ stars. This further supports the recent conclusion of
Kowalski A&A (2010) that DQp stars are DQ stars with pressure distorted Swan
bands. However, we encounter some difficulties in reproducing the IR part of
the SED of stars having a mixed He/H atmosphere. This indicates limitations in
current models of the opacity in dense He/H fluids.Comment: 6 pages, 4 figures, to appear in the proceedings of the "18th
European White Dwarf Workshop" in Krakow, Poland (2012
Towards a standardised line list for G191-B2B, and other DA type objects
We present a comprehensive analysis of the far UV spectrum of G191-B2B over
the range of 900-1700{\AA} using co-added data from the FUSE and STIS archives.
While previous identifications made by Holberg et al. (2003) are reaffirmed in
this work, it is found that many previously unidentified lines can now be
attributed to Fe, Ni, and a few lighter metals. Future work includes extending
this detailed analysis to a wider range of DA objects, in the expectation that
a more complete analysis of their atmospheres can be realised.Comment: 4 pages, 2 figures, 1 table: To appear in the proceedings of the
"18th European White Dwarf Workshop" in Krakow, Poland, 201
Completeness of the Nearby White Dwarf Sample: Let Us Count the Ways
We have recently extended our ongoing survey of the local white dwarf population, effectively doubling the sample volume. Based upon the latest distance determinations, Holberg et al. (2016) estimated the present 20 pc and 25 pc samples were about 86 and 68 percent complete, respectively. Here we examine how the completeness of the 25 pc sample depends upon other observables such as apparent magnitude, proper motion, photometric color index, etc. The results may provide additional clues to why “Sirius-Like systems” are underrepresented in the extended 25 pc sample and how additional nearby single white dwarf stars may be found
Hubble Space Telescope Imaging and Spectroscopy of the Sirius-Like Triple Star System HD 217411
We present Hubble Space Telescope imaging and spectroscopy of HD 217411, a G3
V star associated with the extreme ultraviolet excess source (EUV 2RE
J2300-07.0). This star is revealed to be a triple system with a G 3V primary
(HD 217411 A) separated by ~1.1" from a secondary that is in turn composed of
an unresolved K0 V star (HD 217411 Ba) and a hot DA white dwarf (HD 217411 Bb).
The hot white dwarf dominates the UV flux of the system. However; it is in turn
dominated by the K0 V component beyond 3000 {\AA}. A revised distance of 143 pc
is estimated for the system. A low level photometric modulation having a period
of 0.61 days has also been observed in this system along with a rotational
velocity on the order of 60 km s-1 in the K0 V star. Together both observations
point to a possible wind induced spin up of the K0 V star during the AGB phase
of the white dwarf. The nature of all three components is discussed as are
constraints on the orbits, system age and evolution.Comment: 11 pages, 6 figure
Completeness of the Nearby White Dwarf Sample: Let Us Count the Ways
We have recently extended our ongoing survey of the local white dwarf population, effectively doubling the sample volume. Based upon the latest distance determinations, Holberg et al. (2016) estimated the present 20 pc and 25 pc samples were about 86 and 68 percent complete, respectively. Here we examine how the completeness of the 25 pc sample depends upon other observables such as apparent magnitude, proper motion, photometric color index, etc. The results may provide additional clues to why “Sirius-Like systems” are underrepresented in the extended 25 pc sample and how additional nearby single white dwarf stars may be found
Far-Ultraviolet Spectroscopy of Star-Forming Regions in Nearby Galaxies: Stellar Populations and Abundance Indicators
We present FUSE spectroscopy and supporting data for star-forming regions in
nearby galaxies, to examine their massive-star content and explore the use of
abundance and population indicators in this spectral range for high-redshift
galaxies. New far-ultraviolet spectra are shown for four bright H II regions in
M33 (NGC 588, 592, 595, and 604), the H II region NGC 5461 in M101, and the
starburst nucleus of NGC 7714, supplemented by the very-low-metallicity galaxy
I Zw 18. In each case, we see strong Milky Way absorption systems from H2, but
intrinsic absorption within each galaxy is weak or undetectable, perhaps
because of the "UV bias" in which reddened stars which lie behind
molecular-rich areas are also heavily reddened. We see striking changes in the
stellar-wind lines from these populations with metallicity, suggesting that C
II, C III, C IV, N II, N III, and P V lines are potential tracers of stellar
metallicity in star-forming galaxies. Three of these relations - involving N
IV, C III, and P V - are nearly linear over the range from O/H=0.05--0.8 solar.
The major difference in continuum shapes among these systems is that the giant
H II complex NGC 604 has a stronger continuum shortward of 950 A than any other
object in this sample. Small-number statistics would likely go in the other
direction; we favor this as the result of a discrete star-forming event ~3 Myr
ago, as suggested by previous studies of its stellar population. (Supported by
NASA grant NAG5-8959)Comment: Astronomical Journal, in press (July 2004). 8 figures; before
publication, full-resolution figures are available as a single PDF file from
http://www.astr.ua.edu/keel/fusefigs.pd
Hubble Space Telescope Astrometry of the Procyon System
The nearby star Procyon is a visual binary containing the F5 IV-V subgiant
Procyon A, orbited in a 40.84 yr period by the faint DQZ white dwarf Procyon B.
Using images obtained over two decades with the Hubble Space Telescope, and
historical measurements back to the 19th century, we have determined precise
orbital elements. Combined with measurements of the parallax and the motion of
the A component, these elements yield dynamical masses of 1.478 +/- 0.012 Msun
and 0.592 +/- 0.006 Msun for A and B, respectively.
The mass of Procyon A agrees well with theoretical predictions based on
asteroseismology and its temperature and luminosity. Use of a standard
core-overshoot model agrees best for a surprisingly high amount of core
overshoot. Under these modeling assumptions, Procyon A's age is ~2.7 Gyr.
Procyon B's location in the H-R diagram is in excellent agreement with
theoretical cooling tracks for white dwarfs of its dynamical mass. Its position
in the mass-radius plane is also consistent with theory, assuming a
carbon-oxygen core and a helium-dominated atmosphere. Its progenitor's mass was
1.9-2.2 Msun, depending on its amount of core overshoot.
Several astrophysical puzzles remain. In the progenitor system, the stars at
periastron were separated by only ~5 AU, which might have led to tidal
interactions and even mass transfer; yet there is no direct evidence that these
have occurred. Moreover the orbital eccentricity has remained high (~0.40). The
mass of Procyon B is somewhat lower than anticipated from the
initial-to-final-mass relation seen in open clusters. The presence of heavy
elements in its atmosphere requires ongoing accretion, but the place of origin
is uncertain.Comment: Accepted by Astrophysical Journa
Sub-percent Photometry: Faint DA White Dwarf Spectophotometric Standards for Astrophysical Observatories
We have established a network of 19 faint (16.5 mag 19 mag) northern
and equatorial DA white dwarfs as spectrophotometric standards for present and
future wide-field observatories. Our analysis infers SED models for the stars
that are tied to the three CALSPEC primary standards. Our SED models are
consistent with panchromatic Hubble Space Telescope () photometry to
better than 1%. The excellent agreement between observations and models
validates the use of non-local-thermodynamic-equilibrium (NLTE) DA white dwarf
atmospheres extinguished by interstellar dust as accurate spectrophotometric
references. Our standards are accessible from both hemispheres and suitable for
ground and space-based observatories covering the ultraviolet to the near
infrared. The high-precision of these faint sources make our network of
standards ideally suited for any experiment that has very stringent
requirements on flux calibration, such as studies of dark energy using the
Large Synoptic Survey Telescope (LSST) and the Wide-Field Infrared Survey
Telescope ().Comment: 46 pages, 23 figures, 8 tables, accepted for publication in ApJ