1,455 research outputs found
Pulsations of the Low Mass ZZ Ceti Star HS 1824+6000
Measuring g-mode pulsations of isolated white dwarfs can reveal their
interior properties to high precision. With a spectroscopic mass of ~0.51
M_{\odot} (log g = 7.82), the DAV white dwarf HS 1824+6000 is near the
transition between carbon/oxygen core and helium core white dwarfs, motivating
our photometric search for additional pulsations from the Palomar 60-inch
telescope. We confirmed (with much greater precision) the three frequencies:
2.751190 +/- 0.000010 mHz (363.479 sec), 3.116709 +/- 0.000006 mHz (320.851
sec), 3.495113 +/- 0.000009 mHz (286.114 sec), previously found by B. Voss and
collaborators, and found an additional pulsation at 4.443120 +/- 0.000012 mHz
(225.067 sec). These observed frequencies are similar to those found in other
ZZ Ceti white dwarfs of comparable mass (e.g. log g < 8). We hope that future
observations of much lower mass ZZ Ceti stars (< 0.4 M_{\odot}) will reveal
pulsational differences attributable to a hydrogen covered helium core.Comment: 8 Pages; 4 Figures; Accepted to PASP (scheduled to appear in October
issue
Orphan GRB radio afterglows: Candidates and constraints on beaming
The number of orphan radio afterglows associated with gamma-ray bursts (GRBs)
that should be detected by a flux limited radio survey, is calculated. It is
shown that for jetted GRBs this number is smaller for smaller jet opening angle
(theta), contrary to naive expectation. For a beaming factor
f_b^{-1}=(theta^2/2)^{-1} = 500, roughly the value inferred by Frail et al.
(2001) from analysis of afterglow light curves, we predict that between several
hundreds to several thousands orphan radio afterglows should be detectable
(over all sky) above 1 mJy at GHz frequencies at any given time. This orphan
population is dominated by sources lying at distances of a few hundred Mpc, and
having an age of ~1 yr. A search for point-like radio transients with flux
densities greater than 6 mJy was conducted using the FIRST and NVSS surveys,
yielding a list of 25 orphan candidates. We argue that most of the candidates
are unlikely to be radio supernovae. However, the possibility that they are
radio loud AGNs cannot be ruled out without further observations. Our analysis
sets an upper limit for the all sky number of radio orphans, which corresponds
to a lower limit f_b^{-1}>10 on the beaming factor. Rejection of all candidates
found in our search would imply f_b^{-1}>100. This, and the fact that some
candidates may indeed be radio afterglows, strongly motivate further
observations of these transients.Comment: 18 pages, including 2 figure
The Extreme Hosts of Extreme Supernovae
We use GALEX ultraviolet (UV) and optical integrated photometry of the hosts of 17 luminous supernovae (LSNe, having peak M_V 100 M_ā), by appearing in low-SFR hosts, are potential tests for theories of the initial mass function that limit the maximum mass of a star based on the SFR
Causes and Effects of Corporate Refocusing Programs
We provide evidence that corporate refocusing are motivated, in part, by the desire to enhance shareholder value, but that it is often necessary for agency problems to be reduced before managers will begin divestiture programs. Diversified firms that refocus have significantly greater value losses from their diversification policies than multisegment firms that do not refocus. Major events of market discipline usually must occur, however, before managers attempt to undo suboptimal diversification programs, whereas the same events occur only rarely for a matched sample of nonrefocusing firms during the same time frame. Refocusing firms have a high frequency of CEO changes, and also often have new outside blockholders, unsuccessful takeover bids, and signs of financial distress in the period preceding their divestitures. Finally, we find that the cumulative abnormal returns over all of the refocusing-related announcements of a refocusing firm average 7.3%, and that these abnormal returns are significantly related to the amount of value that was being destroyed by the refocuserās diversification policy
Causes and Effects of Corporate Refocusing Programs
We provide evidence that corporate refocusing are motivated, in part, by the desire to enhance shareholder value, but that it is often necessary for agency problems to be reduced before managers will begin divestiture programs. Diversified firms that refocus have significantly greater value losses from their diversification policies than multisegment firms that do not refocus. Major events of market discipline usually must occur, however, before managers attempt to undo suboptimal diversification programs, whereas the same events occur only rarely for a matched sample of nonrefocusing firms during the same time frame. Refocusing firms have a high frequency of CEO changes, and also often have new outside blockholders, unsuccessful takeover bids, and signs of financial distress in the period preceding their divestitures. Finally, we find that the cumulative abnormal returns over all of the refocusing-related announcements of a refocusing firm average 7.3%, and that these abnormal returns are significantly related to the amount of value that was being destroyed by the refocuserās diversification policy
Two Distant Halo Velocity Groups Discovered by the Palomar Transient Factory
We report the discovery of two new halo velocity groups (Cancer groups A and B) traced by 8 distant RR Lyrae stars and observed by the Palomar Transient Factory (PTF) survey at R.A.~129Ā°, Dec~20Ā° (l~205Ā°, b~32Ā°). Located at 92 kpc from the Galactic center (86 kpc from the Sun), these are some of the most distant substructures in the Galactic halo known to date. Follow-up spectroscopic observations with the Palomar Observatory 5.1-m Hale telescope and W. M. Keck Observatory 10-m Keck I telescope indicate that the two groups are moving away from the Galaxy at v_(gsr) = 78.0+-5.6 km s^(-1) (Cancer group A) and v_(gsr) = 16.3+-7.1 km s^(-1) (Cancer group B). The groups have velocity dispersions of Ļ_(v_)gsr))=12.4+-5.0 km s^(-1) and Ļ _(v_(gsr))=14.9+-6.2 km s^(-1), and are spatially extended (about several kpc) making it very unlikely that they are bound systems, and are more likely to be debris of tidally disrupted dwarf galaxies or globular clusters. Both groups are metal-poor (median metallicities of [Fe/H]^A = -1.6 dex and [Fe/H]^B =-2.1 dex), and have a somewhat uncertain (due to small sample size) metallicity dispersion of ~0.4 dex, suggesting dwarf galaxies as progenitors. Two additional RR Lyrae stars with velocities consistent with those of the Cancer groups have been observed ~25 Ā° east, suggesting possible extension of the groups in that direction
A Revised View of the Transient Radio Sky
We report on a re-analysis of archival data from the Very Large Array for a
sample of ten long duration radio transients reported by Bower and others.
These transients have an implied all-sky rate that would make them the most
common radio transient in the sky and yet most have no quiescent counterparts
at other wavelengths and therefore no known progenitor (other than Galactic
neutron stars). We find that more than half of these transients are due to rare
data artifacts. The remaining sources have lower signal-to-noise ratio (SNR)
than initially reported by 1 to 1.5-sigma. This lowering of SNR matters greatly
since the sources are at the threshold. We are unable to decisively account for
the differences. By two orthogonal criteria one source appears to be a good
detection. Thus the rate of long duration radio transients without optical
counterparts is, at best, comparable to that of the class of recently
discovered Swift J1644+57 nuclear radio transients. We revisit the known and
expected classes of long duration radio transients and conclude that the
dynamic radio sky remains a rich area for further exploration. Informed by the
experience of past searches for radio transients, we suggest that future
surveys pay closer attention to rare data errors and ensure that a wealth of
sensitive multi-wavelength data be available in advance of the radio
observations and that the radio searches should have assured follow-up
resources.Comment: ApJ, submitte
Tracing the Orphan Stream to 55 kpc with RR Lyrae Stars
We report positions, velocities and metallicities of 50 ab-type RR Lyrae
(RRab) stars observed in the vicinity of the Orphan stellar stream. Using about
30 RRab stars classified as being likely members of the Orphan stream, we study
the metallicity and the spatial extent of the stream. We find that RRab stars
in the Orphan stream have a wide range of metallicities, from -1.5 dex to -2.7
dex. The average metallicity of the stream is -2.1 dex, identical to the value
obtained by Newberg et al. (2010) using blue horizontal branch stars. We find
that the most distant parts of the stream (40-50 kpc from the Sun) are about
0.3 dex more metal-poor than the closer parts (within ~30 kpc), suggesting a
possible metallicity gradient along the stream's length. We have extended the
previous studies and have mapped the stream up to 55 kpc from the Sun. Even
after a careful search, we did not identify any more distant RRab stars that
could plausibly be members of the Orphan stream. If confirmed with other
tracers, this result would indicate a detection of the end of the leading arm
of the stream. We have compared the distances of Orphan stream RRab stars with
the best-fit orbits obtained by Newberg et al. (2010). We find that model 6 of
Newberg et al. (2010) cannot explain the distances of the most remote Orphan
stream RRab stars, and conclude that the best fit to distances of Orphan stream
RRab stars and to the local circular velocity is provided by potentials where
the total mass of the Galaxy within 60 kpc is M_{60}~2.7x10^{11} Msun, or about
60% of the mass found by previous studies. More extensive modelling that would
consider non-spherical potentials and the possibility of misalignment between
the stream and the orbit, is highly encouraged.Comment: Submitted to ApJ, 15 pages in emulateapj format, three tables in
machine-readable format (download "Source" from "Other formats"
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