We have studied the interstellar extinction in a field of ∼3 arcmin × 3 arcmin at the core of the 30 Doradus nebula, including the central R 136 cluster, in the Large Magellanic Cloud. Observations ...at optical and near-infrared wavelengths, obtained with the WFC 3 camera on board the Hubble Space Telescope, show that the stars belonging to the red giant clump are spread across the colour–magnitude diagrams because of the considerable and uneven levels of extinction in this region. Since these stars share very similar physical properties and are all at the same distance, they allow us to derive the absolute extinction in a straightforward and reliable way. Thus, we have measured the extinction towards about 180 objects and the extinction law in the range 0.3–1.6 μm. At optical wavelengths, the extinction curve is almost parallel to that of the diffuse Galactic interstellar medium. Taking the latter as a template, the value of R
V
= 4.5 ± 0.2 that we measure indicates that in the optical there is an extra grey component due to a larger fraction of large grains. At wavelengths longer than ∼1 μm, the contribution of this additional component tapers off as λ−1.5, like in the Milky Way, suggesting that the nature of the grains is otherwise similar to those in our Galaxy, but with a ∼2.2 times higher fraction of large grains. These results are consistent with the addition of ‘fresh’ large grains by supernova explosions, as recently revealed by Herschel and ALMA (Atacama Large Millimeter Array) observations of SN 1987A.
Abstract
We report on the properties of the low-mass stars that recently formed in the central
of 30 Dor, including the R136 cluster. Using the photometric catalog of De Marchi et al., based on ...observations with the
Hubble Space Telescope
, and the most recent extinction law for this field, we identify
bona fide pre-main-sequence (PMS) stars showing
excess emission at the
level with an
equivalent width of 20 Å or more. We find a wide spread in age spanning the range
. We also find that the older PMS objects are placed in front of the R136 cluster and are separated from it by a conspicuous amount of absorbing material, indicating that star formation has proceeded from the periphery into the interior of the region. We derive physical parameters for all PMS stars, including masses
m
, ages
t
, and mass accretion rates
. To identify reliable correlations between these parameters, which are intertwined, we use a multivariate linear regression fit of the type
. The values of
a
and
b
for 30 Dor are compatible with those found in NGC 346 and NGC 602. We extend the fit to a uniform sample of
PMS stars with
/
< 1.5 and
in six star-forming regions in the Large and Small Magellanic Clouds and Milky Way with metallicities in the range of 0.1–1.0
. We find
and
. The residuals are systematically different between the six regions and reveal a strong correlation with metallicity
Z
, of the type
. A possible interpretation of this trend is that when the metallicity is higher so is the radiation pressure, and this limits the accretion process, in both its rate and duration.
Recovery of the supernova (SN) delay-time distribution (DTD) - the SN rate versus time that would follow a hypothetical brief burst of star formation - can shed light on SN progenitors and physics, ...as well as on the time-scales of chemical enrichment. Previous attempts to reconstruct the DTD have been based either on comparison of mean SN rates versus redshift to cosmic star-formation history (SFH), or on the comparison of SN rates among galaxies with different mean ages. Here, we present an approach to recover the SN DTD that avoids the averaging and loss of information of other schemes. We compare the SFHs of individual galaxies to the numbers of SNe discovered by a survey in each galaxy (generally zero, sometimes one SN, rarely a few). We apply the method to a subsample of 3505 galaxies, hosting 82 type-Ia SNe (SNe Ia) and 119 core-collapse supernovae (CC SNe), from the Lick Observatory Supernova Search (LOSS), that have SFHs reconstructed from Sloan Digital Sky Survey (SDSS) spectra. We find a >2σ SN Ia DTD signal in our shortest-delay, 'prompt' bin at <420 Myr. We identify and study a systematic error, due to the limited aperture of the SDSS spectroscopic fibres, that causes some of the prompt signal to leak to the later bins of the DTD. After accounting for this systematic error, we demonstrate that a prompt SN Ia contribution is required by the data at the >99 per cent confidence level. We further find a 4σ indication of SNe Ia that are 'delayed' by >2.4 Gyr. Thus, the data support the existence of both prompt and delayed SNe Ia. We measure the time integral over the SN DTD. For CC SNe we find a total yield of 0.010 ± 0.002 SNe per M⊙ formed, in excellent agreement with expectations, if all stars more massive than 8 M⊙ lead to visible SN explosions. This argues against scenarios in which the minimum mass for core-collapse SNe is ≳10 M⊙, or in which a significant fraction of massive stars collapse without an accompanying explosion. For SNe Ia, the time-integrated yield is 0.0023 ± 0.0006 SNe per M⊙ formed, most of them with delays <2.4 Gyr. Finally, we show the robust performance of the method on simulated samples, and demonstrate that its application to already existing SN samples, such as the full LOSS sample, but with complete and unbiased SFH estimates for the survey galaxies, could provide an accurate and detailed measurement of the SN Ia DTD.
Abstract
We report on the accretion properties of low-mass stars in the LH 95 association within the Large Magellanic Cloud. Using noncontemporaneous wideband optical and narrowband H
α
photometry ...obtained with the
Hubble Space Telescope
, we identify 245 low-mass pre-main-sequence (PMS) candidates showing H
α
excess emission above the 4
σ
level. We derive their physical parameters, including effective temperatures, luminosities, masses (
M
⋆
), ages, accretion luminosities, and mass accretion rates (
). We identify two different stellar populations: younger than ∼8 Myr with median
yr
−1
(and
M
⋆
∼ 0.15–1.8
M
⊙
) and older than ∼8 Myr with median
yr
−1
(and
M
⋆
∼ 0.6–1.2
M
⊙
). We find that the younger PMS candidates are assembled in groups around Be stars, while older PMS candidates are uniformly distributed within the region without evidence of clustering. We find that
in LH 95 decreases with time more slowly than what is observed in Galactic star-forming regions (SFRs). This agrees with the recent interpretation, according to which higher metallicity limits the accretion process in both rate and duration due to higher radiation pressure. The
–
M
⋆
relationship shows different behavior at different ages, becoming progressively steeper at older ages, indicating that the effects of mass and age on
cannot be treated independently. With the aim to identify reliable correlations between mass, age, and
, we used a multivariate linear regression fit between these parameters for our PMS candidates. The comparison between our results and those obtained in other SFRs of our Galaxy and the Magellanic Clouds confirms the importance of the metallicity for the study of the
evolution in clusters with different environmental conditions.
We have studied the properties of the stellar populations in the field of the NGC 346 cluster in the Small Magellanic Cloud, using the results of a novel self-consistent method that provides a ...reliable identification of pre-main sequence (PMS) objects actively undergoing mass accretion, regardless of their age. The 680 identified bona fide PMS stars show a bimodal age distribution, with two roughly equally numerous populations peaked, respectively, at ~1 Myr and ~20 Myr. We use the age and other physical properties of these PMS stars to study how star formation has proceeded across time and space in NGC 346. We find no correlation between the locations of young and old PMS stars, nor do we find a correspondence between the positions of young PMS stars and those of massive OB stars of similar age. Furthermore, the mass distribution of stars with similar age shows large variations throughout the region. We conclude that, while on a global scale it makes sense to talk about an initial mass function, this concept is not meaningful for individual star-forming regions. An interesting implication of the separation between regions where massive stars and low-mass objects appear to form is that high-mass stars might not be 'perfect' indicators of star formation and hence a large number of low-mass stars formed elsewhere might have so far remained unnoticed. For certain low surface density galaxies this way of preferential low-mass star formation may be the predominant mechanism, with the consequence that their total mass as derived from the luminosity may be severely underestimated and that their evolution is not correctly understood.
We have studied the properties of the stellar populations in the field of the NGC 346 cluster in the Small Magellanic Cloud, using a novel self-consistent method that allows us to reliably identify ...pre-main-sequence (PMS) objects actively undergoing mass accretion, regardless of their age. The method does not require spectroscopy and combines broadband V and I photometry with narrowband H Delta *a imaging to identify all stars with excess H Delta *a emission and derive the accretion luminosity L acc and mass accretion rate for all of them. The application of this method to existing Hubble Space Telescope (HST)/Advanced Camera for Surveys photometry of the NGC 346 field has allowed us to identify and study 680 bona fide PMS stars with masses from ~0.4 M to ~4 M and ages in the range from ~1 Myr to ~30 Myr. Previous investigations of this region, based on the same data, had identified young (~3 Myr old) candidate PMS stars on the basis of their broadband colors. In this study, we show that there are at least two, almost equally numerous, young populations with distinct ages of, respectively, ~1 and ~20 Myr. We provide accurate physical parameters for all of them. We take advantage of the unprecedented size of our PMS sample and of its spread in mass and age to study the evolution of the mass accretion rate as a function of stellar parameters. We find that, regardless of stellar mass, the mass accretion rate decreases with roughly the square root of the age, or about three times slower than predicted by current models of viscous disk evolution, and that more massive stars systematically have a higher mass accretion rate in proportion to their mass. A multivariate linear regression fit reveals that , where t is the age of the star, m is its mass, and c is a quantity that is higher at lower metallicity. This result is consistent with measurements of the mass accretion rate in the 30 Dor region and in the Milky Way and suggests that the longer duration for mass accretion could be related to lower metallicity. The high-mass accretion rates that we find suggest that a considerable amount of mass is accreted during the PMS phase, of order ~0.2 M or possibly ~20% of the final mass for stars with mass m < 1 M if their disks are eroded by 20 Myr, i.e., before they reach the main sequence. Therefore, PMS evolutionary models that do not account for this effect will systematically underestimate the true age when compared with the observations.
The story of the SN 1987A explosion is briefly reviewed. Although this supernova was somewhat peculiar, the study of SN 1987A has clarified quite a number of important aspects of the nature and the ...properties of supernovae, such as the confirmation of the core collapse of a massive star as the cause of the explosion, as well the confirmation that the decays 56Ni–56Co–56Fe at early times and 44Ti–44Sc at late times, are the main sources of the energy radiated by the ejecta. Still we have not been able to ascertain whether the progenitor was a single star or a binary system, nor have we been able to detect the stellar remnant, a neutron star that should be produced in the core collapse process.
Study of radio supernovae over the past 20 years includes two dozen detected
objects and more than 100 upper limits. From this work it is possible to
identify classes of radio properties, demonstrate ...conformance to and deviations
from existing models, estimate the density and structure of the circumstellar
material and, by inference, the evolution of the presupernova stellar wind, and
reveal the last stages of stellar evolution before explosion. It is also
possible to detect ionized hydrogen along the line of sight, to demonstrate
binary properties of the stellar system, and to show clumpiness of the
circumstellar material. More speculatively, it may be possible to provide
distance estimates to radio supernovae.
Over the past four years the afterglow of gamma-ray bursters has
occasionally been detected in the radio, as well in other wavelengths bands. In
particular, the interesting and unusual gamma-ray burst GRB980425, thought to
be related to SN1998bw, is a possible link between supernovae and gamma-ray
bursters. Analyzing the extensive radio emission data avaliable for SN1998bw,
one can describe its time evolution within the well-established framework
available for the analysis of radio emission from supernovae. This allows
relatively detailed description of a number of physical properties of the
object. The radio emission can best be explained as the interaction of a mildly
relativistic (Γ ∼ 1.6) shock with a dense preexplosion stellar
wind-established circumstellar medium that is highly structured both
azimuthally, in clumps or filaments, and radially, with observed density
enhancements. Because of its unusual characteristics for a Type Ib/c supernova,
the relation of SN1998bw to GRB980425 is strengthened and suggests that at
least some classes of GRBs originate in massive star explosions. Thus,
employing the formalism for describing the radio emission from supernovae and
following the link through SN1998bw/GRB980425, it is possible to model the
gross properties of the radio and optical/infrared emission from the half-dozen
GRBs with extensive radio observations. From this we conclude that at least
some members of the "slow-soft" class of GRBs can be attributed to
the explosion of a massive star in a dense, highly structured circumstellar
medium that was presumably established by the preexplosion stellar system.
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10.
Massive-Star Supernovae as Major Dust Factories Sugerman, Ben E.K; Ercolano, Barbara; Barlow, M.J ...
Science (American Association for the Advancement of Science),
07/2006, Volume:
313, Issue:
5784
Journal Article
Peer reviewed
Open access
We present late-time optical and mid-infrared observations of the Type II supernova 2003gd in the galaxy NGC 628. Mid-infrared excesses consistent with cooling dust in the ejecta are observed 499 to ...678 days after outburst and are accompanied by increasing optical extinction and growing asymmetries in the emission-line profiles. Radiative-transfer models show that up to 0.02 solar masses of dust has formed within the ejecta, beginning as early as 250 days after outburst. These observations show that dust formation in supernova ejecta can be efficient and that massive-star supernovae could have been major dust producers throughout the history of the universe.
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