22,903 research outputs found
The response of self-graviting protostellar discs to slow reduction in cooling timescale: the fragmentation boundary revisited
A number of previous studies of the fragmentation of self-gravitating
protostellar discs have modeled radiative cooling with a cooling timescale
(t_{cool}) parameterised as a simple multiple (beta_{cool}) of the local
dynamical timescale. Such studies have delineated the `fragmentation boundary'
in terms of a critical value of beta_{cool} (beta_{crit}), where the disc
fragments if beta_{cool} < beta_{crit}. Such an approach however begs the
question of how in reality a disc could ever be assembled with beta_{cool} <
beta_{crit}. Here we adopt the more realistic approach of gradually reducing
beta_{cool}, as might correspond to changes in thermal regime due to secular
changes in the disc density profile. We find that when beta_{cool} is gradually
reduced (on a timescale longer than t_{cool}), the disc is stabilised against
fragmentation, compared with models in which beta_{cool} is reduced rapidly. We
therefore conclude that a disc's ability to remain in a self-regulated,
self-gravitating state (without fragmentation) is partly dependent on its
thermal history, as well as its current cooling rate. Nevertheless, a slow
reduction in t_{cool} appears only to lower the fragmentation boundary by about
a factor two in t_{cool} and thus only permits maximum alpha values
(parameterising the efficiency of angular momentum transfer in the disc) that
are about a factor two higher than determined hitherto. Our results therefore
do not undermine the notion of a fundamental upper limit to the heating rate
that can be delivered by gravitational instabilities before the disc is subject
to fragmentation. An important implication of this work, therefore, is that
self-gravitating discs can enter into the regime of fragmentation via secular
evolution and it is not necessary to invoke rapid (impulsive) events to trigger
fragmentation.Comment: accepted for publication in MNRA
GENDER'S ROLE IN MANUSCRIPT ACCEPTANCE: SEX IN THE JOURNAL
Women authors fare poorly at the hands of referees in some economics journals, especially when the review process is not blind. Using data on the 155 manuscripts submitted to the NJARE for publication during the period 1984-88, we found no evidence of differential referee acceptance rates for manuscripts with female and male lead authors.Teaching/Communication/Extension/Profession,
Variation in size of living articulated brachiopods with latitude and depth
Geographical variations in animal characters are one of the main subjects for study in macroecology. Variation with latitude has received special interest. Articulated brachiopods are possibly the commonest macrofossil with large variations in size of taxa through the fossil record. Here, we investigate trends in size of the 3 main orders of articulated brachiopod with latitude and depth. Data were insufficient to identify patterns in Thecideida (a micromorph taxon only recorded from low latitudes). Rhynchonellida had no clear trends in size with latitude or depth. Terebratulida exhibited hemispheric differences in size relations, with increasing length of species towards the pole in the south and no significant trend in the north. Tropical species were small (<20 mm length between 10°N and 10°S), and the largest species were found between 30° and 60° latitude in both hemispheres. There were no articulated brachiopods recorded from the high arctic, and support for a continuous trend in size with latitude was small or absent. In Terebratulida, there was a significant decrease in species length with depth of 1.7 mm per 100 m depth increase. These trends could be explained by competition for space and reduced availability of habitat with progressive depth beyond the continental shelf
Hubble Space Telescope Constraints on the Winds and Astrospheres of Red Giant Stars
We report on an ultraviolet spectroscopic survey of red giants observed by
the Hubble Space Telescope, focusing on spectra of the Mg II h & k lines near
2800 A in order to study stellar chromospheric emission, winds, and
astrospheric absorption. We focus on spectral types between K2 III and M5 III,
a spectral type range with stars that are noncoronal, but possessing strong,
chromospheric winds. We find a very tight relation between Mg II surface flux
and photospheric temperature, supporting the notion that all K2-M5 III stars
are emitting at a basal flux level. Wind velocities (V_w) are generally found
to decrease with spectral type, with V_w decreasing from ~40 km/s at K2 III to
~20 km/s at M5 III. We find two new detections of astrospheric absorption, for
Sigma Pup (K5 III) and Gamma Eri (M1 III). This absorption signature had
previously only been detected for Alpha Tau (K5 III). For the three
astrospheric detections the temperature of the wind after the termination shock
correlates with V_w, but is lower than predicted by the Rankine-Hugoniot shock
jump conditions, consistent with the idea that red giant termination shocks are
radiative shocks rather than simple hydrodynamic shocks. A full hydrodynamic
simulation of the Gamma Eri astrosphere is provided to explore this further.Comment: 16 pages, 8 figures, to appear in The Astrophysical Journa
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