317 research outputs found
Intraspecific functional trait response to advanced snowmelt suggests increase of growth potential but decrease of seed production in snowbed plant species
In ecological theory, it is currently unclear if intraspecific trait responses to environmental variation are shared across plant species. We use one of the strongest environmental variations in alpine ecosystems, i.e., advanced snowmelt due to climate warming, to answer this question for alpine snowbed plants. Snowbeds are extreme habitats where long-lasting snow cover represents the key environmental factor affecting plant life. Intraspecific variation in plant functional traits is a key to understanding the performance and vulnerability of species in a rapidly changing environment. We sampled snowbed species after an above-average warm winter to assess their phenotypic adjustment to advanced snowmelt, based on differences in the natural snowmelt dynamics with magnitudes reflecting predicted future warming. We measured nine functional traits related to plant growth and reproduction in seven vascular species, comparing snowbeds of early and late snowmelt across four snowbed sites in the southern Alps in Italy. The early snowbeds provide a proxy for the advanced snowmelt caused by climatic warming. Seed production was reduced under advanced snowmelt in all seed-forming snowbed species. Higher specific leaf area (SLA) and lower leaf dry matter content (LDMC) were indicative of improved growth potential in most seed-forming species under advanced snowmelt. We conclude, first, that in the short term, advanced snowmelt can improve snowbed species’ growth potential. However, in the long term, results from other studies hint at increasing competition in case of ongoing improvement of conditions for plant growth under continued future climate warming, representing a risk for snowbed species. Second, a lower seed production can negatively affect the seed rain. A reduction of propagule pressure can be crucial in a context of loss of the present snowbed sites and the formation of new ones at higher altitudes along with climate warming. Finally, our findings encourage using plant functional traits at the intraspecific level across species as a tool to understand the future ecological challenges of plants in changing environments
Young core collapse supernova remnants and their supernovae
Massive star supernovae can be divided into four categories depending on the
amount of mass loss from the progenitor star and the star's radius: red
supergiant stars with most of the H envelope intact (SN IIP), stars with some H
but most lost (IIL, IIb), stars with all H lost (Ib, Ic), and blue supergiant
stars with a massive H envelope (SN 1987A-like). Various aspects of the
immediate aftermath of the supernova are expected to develop in different ways
depending on the supernova category: mixing in the supernova, fallback on the
central compact object, expansion of any pulsar wind nebula, interaction with
circumstellar matter, and photoionization by shock breakout radiation. The
observed properties of young supernova remnants allow many of them to be placed
in one of the supernova categories; all the categories are represented except
for the SN 1987A-like type. Of the remnants with central pulsars, the pulsar
properties do not appear to be related to the supernova category. There is no
evidence that the supernova categories form a mass sequence, as would be
expected in a single star scenario for the evolution. Models for young pulsar
wind nebulae expanding into supernova ejecta indicate initial pulsar periods of
10-100 ms and approximate equipartition between particle and magnetic energies.
Ages are obtained for pulsar nebulae, including an age of 2400 pm 500 yr for
3C58, which is not consistent with an origin in SN 1181. There is no evidence
that mass fallback plays a role in neutron star properties.Comment: 43 pages, ApJ, revised, discussion of 3C58 changed, in press for Feb.
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Theoretical Black Hole Mass Distributions
We derive the theoretical distribution function of black hole masses by
studying the formation processes of black holes. We use the results of recent
2D simulations of core-collapse to obtain the relation between remnant and
progenitor masses and fold it with an initial mass function for the
progenitors. We examine how the calculated black-hole mass distributions are
modified by (i) strong wind mass loss at different evolutionary stages of the
progenitors, and (ii) the presence of close binary companions to the black-hole
progenitors. Thus, we are able to derive the binary black hole mass
distribution. The compact remnant distribution is dominated by neutron stars in
the mass range 1.2-1.6Msun and falls off exponentially at higher remnant
masses. Our results are most sensitive to mass loss from winds which is even
more important in close binaries. Wind mass-loss causes the black hole
distribution to become flatter and limits the maximum possible black-hole mass
(<10-15Msun). We also study the effects of the uncertainties in the explosion
and unbinding energies for different progenitors. The distributions are
continuous and extend over a broad range. We find no evidence for a gap at low
values (3-5Msun) or for a peak at higher values (~7Msun) of black hole masses,
but we argue that our black hole mass distribution for binaries is consistent
with the current sample of measured black-hole masses in X-ray transients. We
discuss possible biases against the detection or formation of X-ray transients
with low-mass black holes. We also comment on the possibility of black-hole
kicks and their effect on binaries.Comment: 22 pages, submitted to Ap
The most massive progenitors of neutron stars: CXO J164710.2-455216
The evolution leading to the formation of a neutron star in the very young
Westerlund 1 star cluster is investigated. The turnoff mass has been estimated
to be 35 Msun, indicating a cluster age ~ 3-5 Myr. The brightest X-ray source
in the cluster, CXO J164710.2-455216, is a slowly spinning (10 s) single
neutron star and potentially a magnetar. Since this source was argued to be a
member of the cluster, the neutron star progenitor must have been very massive
(M_zams > 40 Msun) as noted by Muno et al. (2006). Since such massive stars are
generally believed to form black holes (rather than neutron stars), the
existence of this object poses a challenge for understanding massive star
evolution. We point out while single star progenitors below M_zams < 20 Msun
form neutron stars, binary evolution completely changes the progenitor mass
range. In particular, we demonstrate that mass loss in Roche lobe overflow
enables stars as massive as 50-80 Msun, under favorable conditions, to form
neutron stars. If the very high observed binary fraction of massive stars in
Westerlund 1 (> 70 percent) is considered, it is natural that CXO
J164710.2-455216 was formed in a binary which was disrupted in a supernova
explosion such that it is now found as a single neutron star. Hence, the
existence of a neutron star in a given stellar population does not necessarily
place stringent constraints on progenitor mass when binary interactions are
considered. It is concluded that the existence of a neutron star in Westerlund
1 cluster is fully consistent with the generally accepted framework of stellar
evolution.Comment: 5 pages of text and 4 figures (submitted to Astrophysical Journal
Binaries Like to be Twins: Implications for Doubly Degenerate Binaries, the Supernova Ia Rate and Other Interacting Binaries
The recent sample of 21 detached eclipsing binaries in the Small Magellanic
Cloud (Harries et al. 2003, Hilditch et al. 2005) provides a valuable test of
the binary mass function for massive stars. We show that 50% of detached
binaries have companions with very similar masses, q=M_2/M_1 > 0.87, where M_1,
M_2 denote the masses of the two binary components, M_1 > M_2. A Salpeter
relative mass function for the secondary is very strongly excluded, and the
data is consistent with a flat mass function containing 55% of the systems and
a ``twin'' population with q>0.95 containing the remainder. We also survey the
vast existing literature discussing the mass ratio in binaries and conclude
that a significant twin population (of more than 20-25%) exists in binaries
that are likely to interact across a broad range of stellar masses and
metallicity. Interactions involving nearly equal mass stars have distinctly
different properties than those involving stars of unequal mass; the
secondaries will tend to be evolved and the common envelope evolution is
qualitatively different. The implications of such a population for both binary
interactions and star formation are substantial, and we present some examples.
We argue that twin systems may provide a natural stellar population to explain
the recently proposed prompt channel for type Ia SN, and the presence of a twin
population dramatically reduces the maximum inferred NS+BH merger rate relative
to the NS+NS merger rate. Twins may also be important for understanding the
tendency of WD and NS binaries to be nearly equal in mass, and inclusion of
twins in population studies will boost the blue straggler production rate.Comment: 14 pages, 2 figures, ApJL, submitte
Late Emission from the Type Ib/c SN 2001em: Overtaking the Hydrogen Envelope
The Type Ib/c supernova SN 2001em was observed to have strong radio, X-ray,
and Halpha emission at an age of about 2.5 yr. Although the radio and X-ray
emission have been attributed to an off-axis gamma-ray burst, we model the
emission as the interaction of normal SN Ib/c ejecta with a dense, massive (3
Msun) circumstellar shell at a distance about 7 x 10^{16} cm. We investigate
two models, in which the circumstellar shell has or has not been overtaken by
the forward shock at the time of the X-ray observation. The circumstellar shell
was presumably formed by vigorous mass loss with a rate (2-10) x 10^{-3}
Msun/yr at 1000-2000 yr prior to the supernova explosion. The hydrogen envelope
was completely lost, and subsequently was swept up and accelerated by the fast
wind of the presupernova star up to a velocity of 30-50 km/s. Although
interaction with the shell can explain most of the late emission properties of
SN 2001em, we need to invoke clumping of the gas to explain the low absorption
at X-ray and radio wavelengths.Comment: 26 pages, 4 figures, ApJ submitte
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