1,899 research outputs found
Spitzer White Dwarf Planet Limits
We present preliminary limits on the presence of planets around white dwarf
stars using the IRAC photometer on the Spitzer space telescope. Planets emit
strongly in the mid-infrared which allows their presence to be detected as an
excess at these wavelengths. We place limits of for 8 stars assuming
ages of , and for 23 stars.We describe our survey, present our
results and comment on approaches to improve our methodology.Comment: 4 pages, 3 figures, to appear in Proceedings of 15th European White
Dwarf Worksho
A Spitzer White Dwarf Infrared Survey
We present mid-infrared photometry of 124 white dwarf stars with the Spitzer Space Telescope. Objects were observed simultaneously at 4.5 and 8.0 ÎĽm with sensitivities better than 0.1 mJy. This data set can be used to test models of white dwarf atmospheres in a new wavelength regime, as well as to search for planetary companions and debris disks
Tunneling conduction in graphene/(poly)vinyl alcohol composites
Graphene/(Poly)vinyl alcohol (PVA) composite film with thickness
were synthesized by solidification of a PVA solution comprising of dispersed
graphene nanosheets. The close proximity of the graphene sheets enables the
fluctuation induced tunneling of electrons to occur from one sheet to another.
The dielectric data show that the present system can be simulated to a parallel
resistance-capacitor network. The high frequency exponent of the frequency
variation of the ac conductivity indicates that the charge carriers move in a
two-dimensional space. The sample preparation technique will be helpful for
synthesizing flexible conductors.Comment: 10 pages, 8 figure
The Metallicity Distribution of Intracluster Stars in Virgo
We have used the Hubble Space Telescope\u27s Advanced Camera for Surveys (ACS) to detect and measure ~5300 stars in a single intracluster field in the Virgo Cluster. By performing F606W and F814W photometry on these stars, we have determined their metallicity distribution function and constrained the types of stars present in this portion of Virgo\u27s intracluster space. Based on the small number of stars detected that were brighter than the red giant branch (RGB) tip, we suggest that in this region, Virgo\u27s intracluster stars are mostly old (10 Gyr). Through analysis of the RGB stars themselves, we determine that the population contains the full range of metallicities probed (-2.3 ≤ [M/H] ≤ 0.0). We also present evidence that the younger (≤10 Gyr) component of the population is more metal-rich, with [M/H] \u3e -0.5. The spatial distribution of the most metal-poor stars in the field shows significantly more structure than that of the metal-rich stars, indicating that the intracluster population is not well mixed. We discuss the implications that these observations have for the production of intracluster stars and the dynamical evolution of the Virgo Cluster
Virgo’s Intracluster Globular Clusters as Seen by the Advanced Camera for Surveys
We report the discovery of four candidate intracluster globular clusters (IGCs) in a single deepHST ACS field of the Virgo Cluster. We show that each cluster is roughly spherical, has a magnitude near the peak of the Virgo globular cluster luminosity function, has a radial profile that is best fitted by a King model, and is surrounded by an excess of point sources that have the colors and magnitudes of cluster red giant stars. Despite the fact that two of our IGC candidates have integrated colors redder than the mean of the M87 globular cluster system, we propose that all of the objects are metal-poor, with [M/H] \u3c -1. We show that the tidal radii of our intracluster globular clusters are all larger than the mean for Milky Way clusters and suggest that the clusters have undergone less tidal stress than their Galactic counterparts. Finally, we normalize our globular cluster observations to the luminosity of intracluster stars and derive a value of SN ~ 6 for the specific frequency of Virgo intracluster globular clusters. We use these data to constrain the origins of Virgo\u27s intracluster population and suggest that globular clusters in our intracluster field have a different origin than globular clusters in the vicinity of M87. In particular, we argue that dwarf elliptical galaxies may be an important source of intracluster stars
The White Dwarf Cooling Age of M67
A deep imaging survey covering the entire 23\arcmin diameter of the old
open cluster M67 to has been carried out using the mosaic imager
(UHCam) on the Canada-France-Hawaii Telescope. The cluster color-magnitude
diagram (CMD) can be traced from stars on its giant branch at down
through main sequence stars at least as faint as . Stars this low
in luminosity have masses below . A modest white dwarf (WD)
cooling sequence is also observed commencing slightly fainter than
and, after correction for background galaxy and stellar field contamination,
terminating near . The observed WDs follow quite closely a
theoretical cooling sequence for pure carbon core WDs with
hydrogen-rich atmospheres (DA WDs). The cooling time to an of 14.6 for
such WDs is 4.3 Gyr which we take as the WD cooling age of the cluster. A fit
of a set of isochrones to the cluster CMD indicates a turnoff age of 4.0 Gyr.
The excellent agreement between these results suggests that ages derived from
white dwarf cooling should be considered as reliable as those from other dating
techniques. The WDs currently contribute about 9% of the total cluster mass but
the number seen appears to be somewhat low when compared with the number of
giants observed in the cluster.Comment: 15 pages plus 3 diagrams, minor corrections, Accepted for publication
in the Astrophysical Journal Letters, to be published September 10, 199
Identification of Cool White Dwarfs in the NOAO Deep Wide-Field Survey
We report our search for the cool white dwarfs belonging to the Galactic disk
by extending the NOAO Deep Wide-Field Survey. Narrow-band DDO51 photometry of
the Deep Wide-Field Survey's northern field was obtained using the 4m-Mayall
Telescope and the MOSAIC imager to separate cool white dwarfs from other
stellar types of similar T_eff. Follow-up spectroscopy of four white dwarf
candidates from our photometric search resulted in the discovery of two new
cool white dwarfs as companions to M dwarfs.Comment: 2 pages, 2 figures, to appear in proceedings of the 13th European
Workshop on White Dwarf
Photometric Identification of Cool White Dwarfs
We investigate the use of a narrow-band DDO51 filter for photometric
identification of cool white dwarfs. We report photometric observations of 30
known cool white dwarfs with temperatures ranging from 10,000 K down to very
cool temperatures (<3500 K). Follow-up spectroscopic observations of a sample
of objects selected using this filter and our photometric observations show
that DDO51 filter photometry can help select cool white dwarf candidates for
follow-up multi--object spectroscopy by rejecting 65% of main sequence stars
with the same broad--band colors as the cool white dwarfs. This technique is
not selective enough to efficiently feed single--object spectrographs. We
present the white dwarf cooling sequence using this filter. Our observations
show that very cool white dwarfs form a sequence in the r-DDO vs. r-z
color--color diagram and demonstrate that significant improvements are needed
in white dwarf model atmospheres.Comment: Accepted for publication in A
WIYN Open Cluster Study 1: Deep Photometry of NGC 188
We have employed precise V and I photometry of NGC 188 at WIYN to explore the
cluster luminosity function (LF) and study the cluster white dwarfs (WDs). Our
photometry is offset by V = 0.052 (fainter) from Sandage (1962) and Eggen &
Sandage (1969). All published photometry for the past three decades have been
based on these two calibrations, which are in error by 0.05 +- 0.01. We employ
the Pinsonneault etal (1998) fiducial main sequence to derive a cluster
distance modulus of 11.43 +- 0.08. We report observations that are >= 50%
complete to V = 24.6 and find that the cluster central-field LF peaks at M_I ~
3 to 4. This is unlike the solar neighborhood LF and unlike the LFs of
dynamically unevolved portions of open and globular clusters, which rise
continuously until M_I ~ 9.5. Although we find that >= 50% of the unresolved
cluster objects are multiple systems, their presence cannot account for the
shape of the NGC 188 LF. For theoretical reasons (Terlevich 1987; Vesperini &
Heggie 1997) having to do with the survivability of NGC 188 we believe the
cluster is highly dynamically evolved and that the missing low luminosity stars
are either in the cluster outskirts or have left the cluster altogether. We
identify nine candidate WDs, of which we expect three to six are bona fide
cluster WDs. The luminosities of the faintest likely WD indicates an age
(Bergeron, Wesemael, & Beauchamp 1995) of 1.14 +- 0.09 Gyrs. This is a lower
limit to the cluster age and observations probing to V = 27 or 28 will be
necessary to find the faintest cluster WDs and independently determine the
cluster age. While our age limit is not surprising for this ~6 Gyr old cluster,
our result demonstrates the value of the WD age technique with its very low
internal errors. (abridged)Comment: 26 pages, uuencoded gunzip'ed latex + 16 postscrip figures, to be
published in A
A functional polymorphism of the MAOA gene is associated with neural responses to induced anger control
Aggressiveness is highly heritable. Recent experimental work has linked individual differences in a functional polymorphism of the monoamine oxidase-A gene (MAOA) to anger-driven aggression. Other work has implicated the dorsal ACC (dACC) in cognitive-emotional control and the amygdala in emotional arousal. The present imaging genetics study investigated dACC and amygdala reactivity to induced anger control as a function of MAOA genotype. A research assistant asked 38 healthy male undergraduates to control their anger in response to an insult by a rude experimenter. Men with the low-expression allele showed increased dACC and amygdala activation after the insult, but men with the high-expression allele did not. Both dACC and amygdala activation independently mediated the relationship between MAOA genotype and self-reported anger control. Moreover, following the insult, men with the high-functioning allele showed functional decoupling between the amygdala and dACC, but men with the low-functioning allele did not. These results suggest that heightened dACC and amygdala activation and their connectivity are neuroaffective mechanisms underlying anger control in participants with the low-functioning allele of the MAOA gene
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