29 research outputs found
A comparison of DA white dwarf temperatures and gravities from Lyman and Balmer line studies
We present measurements of the effective temperatures and surface gravities
for a sample of hot DA white dwarfs, using the Lyman line data available from
the HUT, ORFEUS and FUSE far-UV space missions. Comparing the results with
those from the standard Balmer line technique, we find that there is a general
good overall agreement between the two methods. However, significant
differences are found for a number of stars, but not always of a consistent
nature in that sometimes the Balmer temperature exceeds that derived from the
Lyman lines and in other instances is lower. We conclude that, with the latest
model atmosphere calculations, these discrepancies probably do not arise from
an inadequate theoretical treatment of the Lyman lines but rather from
systematic effects in the observation and data reduction processes, which
dominate the statistical errors in these spectra. If these systematic data
reduction effects can be adequately controlled, the Lyman line temperature and
gravity measurements are consistent with those obtained from the Balmer lines
when allowance is made for reasonable observational uncertainties.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Society 14 pages, 13 figure
Spectroscopic and photometric analysis of HS 1136+6646: A hot young DAO+K7V post-common- envelope, pre-cataclysmic variable binary
Copyright © 2004 IOP Publishing / American Astronomical SocietyExtensive photometric and spectroscopic observations have been obtained for HS 1136+6646. The observations reveal a newly formed post–common-envelope binary system containing a hot ~DAO.5 primary and a highly irradiated secondary. HS 1136+6646 is the most extreme example yet of a class of short-period hot H-rich white dwarfs with K–M companion systems such as V471 Tau and Feige 24. HS 1136+6646 is a double-line spectroscopic binary showing emission lines of H I, He II, C II, Ca II, and Mg II, due in part to irradiation of the K7 V secondary by the hot white dwarf. Echelle spectra reveal the hydrogen emission lines to be double-peaked with widths of ~200 km s-1, raising the possibility that emission from an optically thin disk may also contribute. The emission lines are observed to disappear near the inferior conjunction. An orbital period of 0.83607 ± 0.00003 days has been determined through the phasing of radial velocities, emission-line equivalent widths, and photometric measurements spanning a range of 24 months. Radial velocity measurements yield an amplitude of KWD = 69 ± 2 km s-1 for the white dwarf and KK7V = 115 ± 1 km s-1 for the secondary star. In addition to orbital variations, photometric measurements have also revealed a low-amplitude modulation with a period of 113.13 minutes and a semiamplitude of 0.0093 mag. These short-period modulations are possibly associated with the rotation of the white dwarf. From fits of the Balmer line profiles, the white dwarf is estimated to have an effective temperature and gravity of ~70,000 K and log g ~ 7.75, respectively. However, this optically derived temperature is difficult to reconcile with the far-UV spectrum of the Lyman line region. Far Ultraviolet Spectroscopic Explorer spectra show the presence of O VI absorption lines and a spectral energy distribution whose slope persists nearly to the Lyman limit. The extremely high temperature of the white dwarf, from both optical and UV measurements, indicates that the binary system is one of the earliest post–common-envelope objects known, having an age around 7.7 × 105 yr. Although the spectrum of the secondary star is best represented by a K7 V star, indications are that the star may be overly luminous for its mass.NASAParticle and Astronomy Research Council, UKNS
Statistical properties of the GALEX spectroscopic stellar sample
The GALEX General Data Release 4/5 includes 174 spectroscopic tiles, obtained
from slitless grism observations, for a total of more than 60,000 ultraviolet
spectra. We have determined statistical properties of the sample of GALEX
stars. We have defined a suitable system of spectroscopic indices, which
measure the main mid-UV features at the GALEX low spectral resolution and we
have employed it to determine the atmospheric parameters of of stars in the
range 4500<Teff<9000 K. Our preliminary results indicate that the sample is
formed by a majority of main sequence F- and G-type stars, with metallicity
[M/H]>-1 dex.Comment: 9 pages, 9 figures, accepted for publication in Astrophysics & Space
Science, UV universe special issu
Stellar structure and compact objects before 1940: Towards relativistic astrophysics
Since the mid-1920s, different strands of research used stars as "physics
laboratories" for investigating the nature of matter under extreme densities
and pressures, impossible to realize on Earth. To trace this process this paper
is following the evolution of the concept of a dense core in stars, which was
important both for an understanding of stellar evolution and as a testing
ground for the fast-evolving field of nuclear physics. In spite of the divide
between physicists and astrophysicists, some key actors working in the
cross-fertilized soil of overlapping but different scientific cultures
formulated models and tentative theories that gradually evolved into more
realistic and structured astrophysical objects. These investigations culminated
in the first contact with general relativity in 1939, when J. Robert
Oppenheimer and his students George Volkoff and Hartland Snyder systematically
applied the theory to the dense core of a collapsing neutron star. This
pioneering application of Einstein's theory to an astrophysical compact object
can be regarded as a milestone in the path eventually leading to the emergence
of relativistic astrophysics in the early 1960s.Comment: 83 pages, 4 figures, submitted to the European Physical Journal
Origin and Evolution of Saturn's Ring System
The origin and long-term evolution of Saturn's rings is still an unsolved
problem in modern planetary science. In this chapter we review the current
state of our knowledge on this long-standing question for the main rings (A,
Cassini Division, B, C), the F Ring, and the diffuse rings (E and G). During
the Voyager era, models of evolutionary processes affecting the rings on long
time scales (erosion, viscous spreading, accretion, ballistic transport, etc.)
had suggested that Saturn's rings are not older than 100 My. In addition,
Saturn's large system of diffuse rings has been thought to be the result of
material loss from one or more of Saturn's satellites. In the Cassini era, high
spatial and spectral resolution data have allowed progress to be made on some
of these questions. Discoveries such as the ''propellers'' in the A ring, the
shape of ring-embedded moonlets, the clumps in the F Ring, and Enceladus' plume
provide new constraints on evolutionary processes in Saturn's rings. At the
same time, advances in numerical simulations over the last 20 years have opened
the way to realistic models of the rings's fine scale structure, and progress
in our understanding of the formation of the Solar System provides a
better-defined historical context in which to understand ring formation. All
these elements have important implications for the origin and long-term
evolution of Saturn's rings. They strengthen the idea that Saturn's rings are
very dynamical and rapidly evolving, while new arguments suggest that the rings
could be older than previously believed, provided that they are regularly
renewed. Key evolutionary processes, timescales and possible scenarios for the
rings's origin are reviewed in the light of tComment: Chapter 17 of the book ''Saturn After Cassini-Huygens'' Saturn from
Cassini-Huygens, Dougherty, M.K.; Esposito, L.W.; Krimigis, S.M. (Ed.) (2009)
537-57
A near-infrared spectroscopic search for very-low-mass cool companions to notable DA white dwarfs
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