43 research outputs found
Analysis of a Hubble Space Telescope Search for Red Dwarfs: Limits on Baryonic Matter in the Galactic Halo
We re-examine a deep {\it Hubble Space Telescope} pencil-beam search for red
dwarfs, stars just massive enough to burn Hydrogen. The authors of this search
(Bahcall, Flynn, Gould \& Kirhakos 1994) found that red dwarfs make up less
than 6\% of the galactic halo. First, we extrapolate this result to include
brown dwarfs, stars not quite massive enough to burn hydrogen; we assume a
mass function. Then the total mass of red dwarfs and brown dwarfs
is 18\% of the halo. This result is consistent with microlensing results
assuming a popular halo model. However, using new stellar models and parallax
observations of low mass, low metallicity stars, we obtain much tighter bounds
on low mass stars. We find the halo red dwarf density to be of the halo,
while our best estimate of this value is 0.14-0.37\%. Thus our estimate of the
halo mass density of red dwarfs drops to 16-40 times less than the reported
result of Bahcall et al (1994). For a mass function, this suggests
a total density of red dwarfs and brown dwarfs of 0.25-0.67\% of the
halo, \ie , (0.9-2.5)\times 10^9\msun out to 50 kpc. Such a low result would
conflict with microlensing estimates by the \macho\ group (Alcock \etal
1995a,b).Comment: 13 pages, 2 figures. Figure one only available via fax or snail-mail
To be published in ApJL. fig. 2 now available in postscript. Some minor
changes in dealing with disk forground. Some cosmetic changes. Updated
reference
The results of deep CCD field surveys: Very low mass halo population stars as dark matter
Halo and disk M dwarfs differ significantly in metallicity. Multiâcolor deep CCD surveys are uniquely capable of detecting these separate populations of stars with differing metallicity and therefore colors. Analyzing very deep threeâband CCD images covering 192 arcmin2 at high galactic latitude we find no evidence for a population of extreme low mass M subdwarfs sufficient to account for the halo dark matter. These observations covering a volume of 2Ă105 pc3 are consistent with extrapolations of a halo luminosity function determined using low metallicity stars in the solar neighborhood. © 1995 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87546/2/91_1.pd
Radii of 88 M subdwarfs and updated radius relations for low-metallicity M-dwarf stars
M subdwarfs are low-metallicity M dwarfs that typically inhabit the halo population of the Galaxy. Metallicity controls the opacity of stellar atmospheres; in metal-poor stars, hydrostatic equilibrium is reached at a smaller radius, leading to smaller radii for a given effective temperature. We compile a sample of 88 stars that span spectral classes K7 to M6 and include stars with metallicity classes from solar-metallicity dwarf stars to the lowest metallicity ultra subdwarfs to test how metallicity changes the stellar radius. We fit models to Palomar Double Spectrograph (DBSP) optical spectra to derive effective temperatures (T_ eff) and we measure bolometric luminosities (L_ bol) by combining broad wavelength-coverage photometry with Gaia parallaxes. Radii are then computed by combining the T_ eff and L_ bol using the StefanâBoltzman law. We find that for a given temperature, ultra subdwarfs can be as much as five times smaller than their solar-metallicity counterparts. We present color-radius and color-surface brightness relations that extend down to [Fe/H] of â2.0 dex, in order to aid the radius determination of M subdwarfs, which will be especially important for the WFIRST exoplanetary microlensing survey.Published versio
The Mass-Function of Low Mass Halo Stars: Limits on Baryonic Halo Dark Matter
We derive mass functions (MF) for halo red dwarfs (the faintest hydrogen
burning stars) and then extrapolate to place limits on the total mass of halo
brown dwarfs (stars not quite massive enough to burn hydrogen). The mass
functions are obtained from the luminosity function of a sample of 114 local
halo stars in the USNO parallax survey (Dahn \etal 1995). We use stellar models
of Alexander \etal (1996) and make varying assumptions about metallicity and
about possible unresolved binaries in the sample. We find that the MF for halo
red dwarfs cannot rise more quickly than as one approaches the hydrogen
burning limit. Using recent results from star formation theory, we extrapolate
the MF into the brown-dwarf regime. We see that likely extrapolations imply
that the total mass of brown dwarfs in the halo is less than of the
local mass density of the halo ( for the more realistic models we
consider). Our limits apply to brown dwarfs in the halo that come from the same
stellar population as the red dwarfs.Comment: Significant changes over previous submission. To be published ApJ
Letters, 16 pages, latex, one figur
The Deep Lens Survey Transient Search I : Short Timescale and Astrometric Variability
We report on the methodology and first results from the Deep Lens Survey
transient search. We utilize image subtraction on survey data to yield all
sources of optical variability down to 24th magnitude. Images are analyzed
immediately after acquisition, at the telescope and in near-real time, to allow
for followup in the case of time-critical events. All classes of transients are
posted to the web upon detection. Our observing strategy allows sensitivity to
variability over several decades in timescale. The DLS is the first survey to
classify and report all types of photometric and astrometric variability
detected, including solar system objects, variable stars, supernovae, and short
timescale phenomena. Three unusual optical transient events were detected,
flaring on thousand-second timescales. All three events were seen in the B
passband, suggesting blue color indices for the phenomena. One event (OT
20020115) is determined to be from a flaring Galactic dwarf star of spectral
type dM4. From the remaining two events, we find an overall rate of \eta = 1.4
events deg-2 day-1 on thousand-second timescales, with a 95% confidence limit
of \eta < 4.3. One of these events (OT 20010326) originated from a compact
precursor in the field of galaxy cluster Abell 1836, and its nature is
uncertain. For the second (OT 20030305) we find strong evidence for an extended
extragalactic host. A dearth of such events in the R passband yields an upper
95% confidence limit on short timescale astronomical variability between 19.5 <
R < 23.4 of \eta_R < 5.2. We report also on our ensemble of astrometrically
variable objects, as well as an example of photometric variability with an
undetected precursor.Comment: 24 pages, 12 figures, 3 tables. Accepted for publication in ApJ.
Variability data available at http://dls.bell-labs.com/transients.htm
A New Sample of Cool Subdwarfs from SDSS: Properties and Kinematics
We present a new sample of M subdwarfs compiled from the 7th data release of
the Sloan Digital Sky Survey. With 3517 new subdwarfs, this new sample
significantly increases the number of spectroscopically confirmed low-mass
subdwarfs. This catalog also includes 905 extreme and 534 ultra sudwarfs. We
present the entire catalog including observed and derived quantities, and
template spectra created from co-added subdwarf spectra. We show color-color
and reduced proper motion diagrams of the three metallicity classes, which are
shown to separate from the disk dwarf population. The extreme and ultra
subdwarfs are seen at larger values of reduced proper motion as expected for
more dynamically heated populations. We determine 3D kinematics for all of the
stars with proper motions. The color-magnitude diagrams show a clear separation
of the three metallicity classes with the ultra and extreme subdwarfs being
significantly closer to the main sequence than the ordinary subdwarfs. All
subdwarfs lie below (fainter) and to the left (bluer) of the main sequence.
Based on the average velocities and their dispersions, the extreme
and ultra subdwarfs likely belong to the Galactic halo, while the ordinary
subdwarfs are likely part of the old Galactic (or thick) disk. An extensive
activity analysis of subdwarfs is performed using H emission and 208
active subdwarfs are found. We show that while the activity fraction of
subdwarfs rises with spectral class and levels off at the latest spectral
classes, consistent with the behavior of M dwarfs, the extreme and ultra
subdwarfs are basically flat.Comment: 66 pages, 23 figures, accepted in Ap
On the chains of star complexes and superclouds in spiral arms
The relation is studied between occurrence of a regular chain of star
complexes and superclouds in a spiral arm, and other properties of the latter.
A regular string of star complexes is located in the north-western arm of M31;
they have about the same size 0.6 kpc with spacing of 1.1 kpc. Within the same
arm segment the regular magnetic field with the wavelength of 2.3 kpc was found
by Beck et al. (1989). We noted that this wavelength is twice as large as the
spacing between complexes and suggested that they were formed in result of
magneto-gravitational instability developed along the arm. In this NW arm, star
complexes are located inside the gas-dust lane, whilst in the south-western arm
of M31 the gas-dust lane is upstream of the bright and uniform stellar arm.
Earlier, evidence for the age gradient has been found in the SW arm. All these
are signatures of a spiral shock, which may be associated with unusually large
(for M31) pitch-angle of this SW arm segment. Such a shock may prevent the
formation of the regular magnetic field, which might explain the absence of
star complexes there. Anti-correlation between shock wave signatures and
presence of star complexes is observed in spiral arms of a few other galaxies.
Regular chains of star complexes and superclouds in spiral arms are rare, which
may imply that a rather specific mechanism is involved in their formation, and
the most probable one is the Parker-Jeans instability. The spiral pattern of
our Galaxy is briefly discussed; it may be of M101 type in the outer parts. The
regular bi-modal spacing of HI superclouds is found in Carina and Cygnus
(Outer) arms, which may be an indirect evidence for the regular magnetic field
along these arms.Comment: 20 pages, 12 figures, accepted for publication in MNRA