153 research outputs found
UV-bright stars in globular clusters
This paper highlights globular cluster studies with Ultraviolet Imaging Telescope (UIT) in three areas: the discrepancy between observed ultraviolet HB magnitudes and predictions of theoretical HB models; the discovery of two hot subdwarfs in NGC 1851, a globular not previously known to contain such stars; and spectroscopic follow up of newly identified UV-bright stars in M79 and w Cen. I also present results of a recent observation of NGC 6397 with the Voyager ultraviolet spectrometer
How Do Analyst Recommendations Respond to Major News?
We examine how analysts respond to public information when setting stock recommendations. We model the determinants of analystsâ recommendation changes following large stock price movements. We find evidence of an asymmetry following large positive and negative returns. Following large stock price increases, analysts are equally likely to upgrade
or downgrade. Following large stock price declines, analysts are more likely to downgrade. This asymmetry exists after accounting for investment banking relationships
and herding behavior. This result suggests recommendation changes are âstickyâ in one direction, with analysts reluctant to downgrade. Moreover, this result implies that analystsâ optimistic bias may vary through time
The High Chromospheres of the Late A Stars
We report the detection of N V 1239 A transition region emission in HST/GHRS
spectra of the A7 V stars, Alpha Aql and Alpha Cep. Our observations provide
the first direct evidence of 1-3 x 10^5 K material in the atmospheres of normal
A-type stars. For both stars, and for the mid-A--type star Tau3 Eri, we also
report the detection of chromospheric emission in the Si III 1206 A line. At a
B-V color of 0.16 and an effective temperature of 8200 K, Tau3 Eri becomes the
hottest main sequence star known to have a chromosphere and thus an outer
convection zone. We see no firm evidence that the Si III line surface fluxes of
the A stars are any lower than those of moderately active, solar-type, G and K
stars. This contrasts sharply with their coronal X-ray emission, which is >100
times weaker than that of the later-type stars. Given the strength of the N V
emission observed here, it now appears unlikely that the X-ray faintness of the
A stars is due to their forming very cool, <= 1 MK coronae. An alternative
explanation in terms of mass loss in coronal winds remains a possibility,
though we conclude from moderate resolution spectra of the Si III lines that
such winds, if they exist, do not penetrate into the chromospheric Si
III--forming layers of the star, since the profiles of these lines are *not*
blueshifted, and may well be redshifted with respect to the star.Comment: LaTex, 12 pages, 3 Postscript figures, uses aaspp4, accepted by Ap
Cost of capital and earnings transparency
We provide evidence that firms with more transparent earnings enjoy a lower cost of capital. We base our earnings transparency measure on the extent to which earnings and change in earnings covary contemporaneously with returns. We find a significant negative relation between our transparency measure and subsequent excess and portfolio mean returns, and expected cost of capital, even after controlling for previously documented determinants of cost of capital
The Origin of Hot Subluminous Horizontal-Branch Stars in Omega Centauri and NGC 2808
Hot subluminous stars lying up to 0.7 mag below the extreme horizontal branch
(EHB) are found in the UV color-magnitude diagrams of omega Cen and NGC 2808.
Such stars are unexplained by canonical HB theory. In order to explore the
origin of these subluminous stars, we evolved a set of low-mass stars from the
main sequence through the helium-core flash to the HB for a wide range in the
mass loss along the red-giant branch (RGB). Stars with the largest mass loss
evolve off the RGB to high effective temperatures before igniting helium in
their cores. Our results indicate that the subluminous EHB stars, as well as
the gap within the EHB of NGC 2808, can be explained if these stars undergo a
late helium-core flash on the white-dwarf cooling curve. Under these conditions
the flash convection will penetrate into the stellar envelope, thereby mixing
most, if not all, of the envelope hydrogen into the hot helium- burning
interior. This phenomenon is analogous to the "born-again" scenario for
producing hydrogen-deficient stars during a very late helium-shell flash.
"Flash mixing" greatly enhances the envelope helium and carbon abundances and,
as a result, leads to an abrupt increase in the HB effective temperature. We
argue that the EHB gap in NGC 2808 is caused by this theoretically predicted
dichotomy in the HB morphology. Using new helium- and carbon-rich stellar
atmospheres, we show that the flash-mixed stars have the same reduced UV flux
as the subluminous EHB stars. Moreover, we demonstrate that models without
flash mixing lie, at most, ~0.1 mag below the EHB and hence fail to explain the
observations. Flash mixing may also provide a new evolutionary channel for
producing the high gravity, He-rich sdO and sdB stars.Comment: 8 pages, 5 figures, to appear in "Omega Centauri: a Unique Window
into Astrophysics" (Cambridge, August, 2001), ASP Conf. Ser., edited by F.
van Leeuwen, G. Piotto, and J. Hughe
The Contribution of Bank Regulation and Fair Value Accounting to Procyclical Leverage
Our analytical description of how banksâ responses to asset price changes can result in procyclical leverage reveals that for banks with a binding regulatory leverage constraint, absent differences in regulatory risk weights across assets, leverage is not procyclical. For banks without a binding constraint, fair value and bank regulation both can contribute to procyclical leverage. Empirical findings based on a large sample of US commercial banks reveal that bank regulation explains procyclical leverage for banks relatively close to the regulatory leverage constraint and contributes to procyclical leverage for those that are not. Fair value accounting does not contribute to procyclical leverage
The Discovery of Pulsating Hot Subdwarfs in NGC 2808
We present the results of a Hubble Space Telescope program to search for
pulsating hot subdwarfs in the core of NGC 2808. These observations were
motivated by the recent discovery of such stars in the outskirts of omega Cen.
Both NGC 2808 and omega Cen are massive globular clusters exhibiting complex
stellar populations and large numbers of extreme horizontal branch stars. Our
far-UV photometric monitoring of over 100 hot evolved stars has revealed six
pulsating subdwarfs with periods ranging from 85 to 149 s and UV amplitudes of
2.0 to 6.8%. In the UV color-magnitude diagram of NGC 2808, all six of these
stars lie immediately below the canonical horizontal branch, a region populated
by the subluminous "blue-hook" stars. For three of these six pulsators, we also
have low-resolution far-UV spectroscopy that is sufficient to broadly constrain
their atmospheric abundances and effective temperatures. Curiously, and in
contrast to the omega Cen pulsators, the NGC 2808 pulsators do not exhibit the
spectroscopic or photometric uniformity one might expect from a well-defined
instability strip, although they all fall within a narrow band (0.2 mag) of
far-UV luminosity.Comment: 5 pages, 1 table, 2 color and 2 grayscale figures. Accepted for
publication in The Astrophysical Journal Letter
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