895 research outputs found
The context of the Local Volume: structures and motions in the nearby universe
The 6dF Galaxy Survey (6dFGS) and the 2MASS Redshift Survey (2MRS) provide
the most complete maps of the large-scale structures and motions in the nearby
universe. These maps have been used to reconstruct the density field in the
local volume, and to predict the corresponding velocity field and the dipole of
the Local Group motion.Comment: 4 pages, to appear in "Galaxies in the Local Volume", 2008, eds B.
Koribalski and H. Jerjen, Springer Astrophysics and Space Science Series
(proceedings of conference held in Sydney on 8-13 July 2007
Optical spectroscopy of candidate Alpha Persei white dwarfs
As part of an investigation into the high mass end of the initial mass-final
mass relation we performed a search for new white dwarf members of the nearby
(172.4 pc), young (80-90 Myr) Persei open star cluster. The
photometric and astrometric search using the UKIRT Infrared Deep Sky Survey and
SuperCOSMOS sky surveys discovered 14 new white dwarf candidates. We have
obtained medium resolution optical spectra of the brightest 11 candidates using
the William Herschel Telescope and confirmed that while 7 are DA white dwarfs,
3 are DB white dwarfs and one is an sdOB star, only three have cooling ages
within the cluster age, and from their position on the initial mass-final mass
relation, it is likely none are cluster members. This result is disappointing,
as recent work on the cluster mass function suggests that there should be at
least one white dwarf member, even at this young age. It may be that any white
dwarf members of Per are hidden within binary systems, as is the case
in the Hyades cluster, however the lack of high mass stars within the cluster
also makes this seem unlikely. One alternative is that a significant level of
detection incompleteness in the legacy optical image survey data at this
Galactic latitude has caused some white dwarf members to be overlooked. If this
is the case, Gaia will find them.Comment: 8 pages, 7 Figures, 3 Tables. Accepted for publication in MNRA
The infrared counterpart of the Z source GX5-1
We have obtained UKIRT infrared observations of the field of the bright
Galactic Z source GX5-1. From an astrometric plate solution tied to Tycho-ACT
standards we have obtained accurate positions for the stars in our field which,
combined with an accurate radio position, have allowed us to identify the
probable infrared counterpart of GX5-1. Narrow-band photometry marginally
suggests excess Br-gamma emission in the counterpart, supporting its
association with an accretion-disc source. No significant variability is
observed in a limited number of observations. We compare the H and K magnitudes
with those of other Z sources, and briefly discuss possible sources of infrared
emission in these systems.Comment: 5 pages, 2 figures, accepted by MNRA
Direct Detection of Galactic Halo Dark Matter
The Milky Way Galaxy contains a large, spherical component which is believed
to harbor a substantial amount of unseen matter. Recent observations indirectly
suggest that as much as half of this ``dark matter'' may be in the form of old,
very cool white dwarfs, the remnants of an ancient population of stars as old
as the Galaxy itself. We conducted a survey to find faint, cool white dwarfs
with large space velocities, indicative of their membership in the Galaxy's
spherical halo component. The survey reveals a substantial, directly observed
population of old white dwarfs, too faint to be seen in previous surveys. This
newly discovered population accounts for at least 2% of the halo dark matter.
It provides a natural explanation for the indirect observations, and represents
a direct detection of Galactic halo dark matter.Comment: 13 pages, 4 figures, 1 table. Note added after Science Express online
publication: This text reflects the correction of a few typographical errors
in the online version of the table. It also includes the new constraint on
the calculation of d_max which accounts for the fact that the survey could
not have detected stars with proper motions below 0.33 arcseconds per year.
Published online at ScienceExpress www.sciencemag.org 22 March 2001;
10.1126/science.1059954; To appear in Science 27 April 200
The Solar Neighborhood. XIX. Discovery and Characterization of 33 New Nearby White Dwarf Systems
We present spectra for 33 previously unclassified white dwarf systems
brighter than V = 17 primarily in the southern hemisphere. Of these new
systems, 26 are DA, 4 are DC, 2 are DZ, and 1 is DQ. We suspect three of these
systems are unresolved double degenerates. We obtained VRI photometry for these
33 objects as well as for 23 known white dwarf systems without trigonometric
parallaxes, also primarily in the southern hemisphere. For the 56 objects, we
converted the photometry values to fluxes and fit them to a spectral energy
distribution using the spectroscopy to determine which model to use (i.e. pure
hydrogen, pure helium, or metal-rich helium), resulting in estimates of
effective temperature and distance. Eight of the new and 12 known systems are
estimated to be within the NStars and Catalogue of Nearby Stars (CNS) horizons
of 25 pc, constituting a potential 18% increase in the nearby white dwarf
sample. Trigonometric parallax determinations are underway via CTIOPI for these
20 systems.
One of the DCs is cool so that it displays absorption in the near infrared.
Using the distance determined via trigonometric parallax, we are able to
constrain the model-dependent physical parameters and find that this object is
most likely a mixed H/He atmosphere white dwarf similar to other cool white
dwarfs identified in recent years with significant absorption in the infrared
due to collision-induced absorptions by molecular hydrogen.Comment: 33 pages, 10 figures, accepted for publication in the Astronomical
Journa
The Solar Neighborhood XXVII: Discovery of New Proper Motion Stars with mu > 0.18 "/yr in the Southern Sky with 16.5 > R_59F > 18.0
Here we present 1584 new southern proper motion systems with mu > 0.18 "/yr
and 16.5 > R_59F > 18.0. This search complements the six previous
SuperCOSMOS-RECONS (SCR) proper motion searches of the southern sky for stars
within the same proper motion range, but with R_59F < 16.5. As in previous
papers, we present distance estimates for these systems and find that three
systems are estimated to be within 25 pc, including one, SCR 1546-5534,
possibly within the RECONS 10 pc horizon at 6.7 pc, making it the second
nearest discovery of the searches. We find 97 white dwarf candidates with
distance estimates between 10 and 120 pc, as well as 557 cool subdwarf
candidates. The subdwarfs found in this paper make up nearly half of the
subdwarf systems reported from our SCR searches, and are significantly redder
than those discovered thus far. The SCR searches have now found 155 red dwarfs
estimated to be within 25 pc, including 10 within 10 pc. In addition, 143 white
dwarf candidates and 1155 cool subdwarf candidates have been discovered. The
1584 systems reported here augment the sample of 4724 systems previously
discovered in our SCR searches, and imply that additional systems fainter than
R_59F = 18.0 are yet to be discovered.Comment: 11 pages of text, seven figure
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