244 research outputs found
A New Look at the Local White Dwarf Population
We have conducted a detailed new survey of the local population of white dwarfs lying within 20 pc of the Sun. A new revised catalog of local white dwarfs containing 122 entries (126 individual degenerate stars) is presented. This list contains 27 white dwarfs not included in a previous list from 2002, as well as new and recently published trigonometric parallaxes. In several cases new members of the local white dwarf population have come to light through accurate photometric distance estimates. In addition, a suspected new double degenerate system (WD 0423+120) has been identified. The 20 pc sample is currently estimated to be 80% complete. Using a variety of recent spectroscopic, photometric, and trigonometric distance determinations, we re-compute a space density of 4.8 ± 0.5 Ă 10â3 pcâ3 corresponding to a mass density of 3.2 ± 0.3 Ă 10â3 M pcâ3 from the complete portion of the sample within 13 pc. We find an overall mean mass for the local white dwarfs of 0.665 M, a value larger than most other non-volume-limited estimates. Although the sample is small, we find no evidence of a correlation between mass and temperature in which white dwarfs below 13,000 K are systematically more massive than those above this temperature. Within 20 pc 25% of the white dwarfs are in binary systems (including double degenerate systems). Approximately 6% are double degenerates and 6.5% are Sirius-like systems. The fraction of magnetic white dwarfs in the local population is found to be 13%
Revised physical elements of the astrophysically important O9.5+O9.5V eclipsing binary system Y Cyg
Thanks to its long and rich observational history and rapid apsidal motion,
the massive eclipsing binary Y Cyg represents one of the cornestones to
critical tests of stellar evolution theory for massive stars. Yet, the
determination of the basic physical properties is less accurate than it could
be given the existing number of spectral and photometric observations. Our goal
is to analyze all these data simultaneously with the new dedicated series of
our own spectral and photometric observations from observatories widely
separated in longitude. We obtained new series of UBV observations at three
observatories separated in local time to obtain complete light curves of Y Cyg
for its orbital period close to 3 days. This new photometry was reduced and
carefully transformed to the standard UBV system using the HEC22 program. We
also obtained new series of red spectra secured at two observatories and
re-analyzed earlier obtained blue electronic spectra. Our analyses provide the
most accurate so far published value of the apsidal period of 47.805 +/- 0.030
yrs and the following physical elements: M1=17.72+/-0.35$ Msun, M2=17.73+/-0.30
Msun, R1=5.785+/-0.091 Rsun, and R2=5.816+/-0.063 Rsun. The disentangling thus
resulted in the masses, which are somewhat higher than all previous
determinations and virtually the same for both stars, while the light curve
implies a slighly higher radius and luminosity for star 2. The above empirical
values imply the logarithm of the internal structure constant log k2 = -1.937.
A comparison with Claret's stellar interior models implies an age close to 2
millions yrs for both stars. The claimed accuracy of modern element
determination of 1-2 per cent seems still a bit too optimistic and obtaining
new high-dispersion and high-resolution spectra is desirable.Comment: 13 pages; accepted for publication in Astronomy and Astrophysic
The galactic population of white dwarfs
Original paper can be found at: http://www.iop.org/EJ/conf DOI: 10.1088/1742-6596/172/1/012004 [16th European White Dwarfs Workshop]The contribution of white dwarfs of the different Galactic populations to the stellar content of our Galaxy is only poorly known. Some authors claim a vast population of halo white dwarfs, which would be in accordance with some investigations of the early phases of Galaxy formation claiming a top-heavy initialâ massâ function. Here, I present a model of the population of white dwarfs in the Milky Way based on observations of the local white dwarf sample and a standard model of Galactic structure. This model will be used to estimate the space densities of thin disc, thick disc and halo white dwarfs and their contribution to the baryonic mass budget of the Milky Way. One result of this investigation is that white dwarfs of the halo population contribute a large fraction of the Galactic white dwarf number count, but they are not responsible for the lion's share of stellar mass in the Milky Way. Another important result is the substantial contribution of the â often neglected â population of thick disc white dwarfs. Misclassification of thick disc white dwarfs is responsible for overestimates of the halo population in previous investigations.Peer reviewe
White dwarf cooling and large extra dimensions
Theories of fundamental interactions with large extra dimensions have
recently become very popular. Astrophysical bounds from the Sun, red-giants and
SN1987a have already been derived by other authors for the theory proposed by
Arkani-Hamed, Dimopoulos and Dvali. In this paper we consider G117-B15A
pulsating white dwarf (ZZ Ceti star) for which the secular rate at which the
period of its fundamental mode increases has been accurately measured and
claimed that this mode of G117-B15A is perhaps the most stable oscillation ever
recorded in the optical band. Because an additional channel of energy loss
(Kaluza-Klein gravitons) would speed up the cooling rate, one is able to use
the aforementioned stability to derive a bound on theories with large extra
dimensions. Within the framework of the theory with large extra dimensions
proposed by Arkani-Hamed, Dimopoulos and Dvali we find the lower bound on
string comapctification scale which is more stringent than solar or red-giant
bounds.Comment: 9 pages,LaTeX, new references added. Phys. Rev. D in pres
On the nature of the FBS blue stellar objects and the completeness of the Bright Quasar Survey. II
In Paper I (Mickaelian et al. 1999), we compared the surface density of QSOs
in the Bright Quasar Survey (BQS) and in the First Byurakan Survey (FBS) and
concluded that the completeness of the BQS is of the order of 70% rather than
30-50% as suggested by several authors. A number of new observations recently
became available, allowing a re-evaluation of this completeness. We now obtain
a surface density of QSOs brighter than B = 16.16 in a subarea of the FBS
covering ~2250 deg^2, equal to 0.012 deg^-2 (26 QSOs), implying a completeness
of 53+/-10%.Comment: LaTeX 2e, 11 pages, 3 tables and 3 figures (included in text). To
appear in Astrophysics. Uses a modified aaspp4.sty (my_aaspp4.sty), included
in packag
The Spectral Energy Distributions of White Dwarfs in 47 Tucanae: The Distance to the Cluster
We present a new distance determination to the Galactic globular cluster 47
Tucanae by fitting the spectral energy distributions of its white dwarfs to
pure hydrogen atmosphere white dwarf models. Our photometric dataset is
obtained from a 121 orbit Hubble Space Telescope program using the Wide Field
Camera 3 UVIS/IR channels, capturing F390W, F606W, F110W, and F160W images.
These images cover more than 60 square arcmins and extend over a radial range
of 5-13.7 arcmin (6.5-17.9 pc) within the globular cluster. Using a likelihood
analysis, we obtain a best fitting unreddened distance modulus of (m -
M)o=13.36+/-0.02+/-0.06 corresponding to a distance of 4.70+/-0.04+/-0.13 kpc,
where the first error is random and the second is systematic. We also search
the white dwarf photometry for infrared excess in the F160W filter, indicative
of debris disks or low mass companions, and find no convincing cases within our
sample.Comment: Accepted to The Astronomical Journal, 13 Figures, 2 Tables. Figures 3
and 6 are figure sets, each composed of 59 subfigures (to appear in the
electronic journal). This is a Companion paper to the article ID:
submit/037561
Adrenomedullin blockade improves catecholamine responsiveness and kidney function in resuscitated murine septic shock
Pathways between childhood trauma, intimate partner violence, and harsh parenting : findings from the UN multi-country study on men and violence in Asia and the Pacific
2016-2017 > Academic research: refereed > Publication in refereed journal201804_a bcmaVersion of RecordPublishe
Discovery of fourteen new ZZ Cetis with SOAR
We report the discovery of fourteen new ZZ Cetis with the 4.1m Southern
Astrophysical Research telescope, at Cerro Pachon, in Chile. The candidates
were selected from the SDSS (Sloan Digital Sky Survey) DA white dwarf stars
with Teff obtained from the optical spectra fit, inside the ZZ Ceti instability
strip. Considering these stars are multi-periodic pulsators and the pulsations
propagate to the nucleus of the star, they carry information on the structure
of the star and evolution of the progenitors. The ZZ Cetis discovered till 2003
are mainly within 100 pc from the Sun, and probe only the solar vicinity. The
recently discovered ones, and those reported here, may sample a distinct
population as they were selected mainly perpendicular to the galactic disk and
cover a distance up to ~400pc.Comment: 12 pages, 2 figure
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