70 research outputs found
The Star Formation History of the Large Magellanic Cloud
Using WFPC2 aboard the Hubble Space Telescope, we have created deep
color-magnitude diagrams in the V and I passbands for approximately 100,000
stars in a field at the center of the LMC bar and another in the disk. The
main--sequence luminosity functions (LFs) from 19 mag < V < 23.5 mag, the red
clump and horizontal branch morphologies, and the differential Hess diagram of
the two fields all strongly imply that the disk and bar have significantly
different star-formation histories (SFHs). The disk's SFH has been relatively
smooth and continuous over the last 15 Gyr while the bar's SFH was dominated by
star formation episodes at intermediate ages. Comparison of the LF against
predictions based on Padova theoretical stellar evolution models and an assumed
age-metallicity relationship allows us to identify the dominant stellar
populations in the bar with episodes of star formation that occurred from 4 to
6 and 1 to 2 Gyr ago. These events accounted, respectively, for approximately
25% and 15% of its stellar mass. The disk field may share a mild enhancement in
SF for the younger episode, and thus we identify the 4 to 6 Gyr episode with
the formation of the LMC bar.Comment: 14 pages, 5 figures, Latex, also available at
http://www.ps.uci.edu/physics/smeckerhane.html. Accepted for publication in
Ap
Uniformity of V minus Near Infrared Color Evolution of Type Ia Supernovae, and Implications for Host Galaxy Extinction Determination
From an analysis of SNe 1972E, 1980N, 1981B, 1981D, 1983R, 1998bu, 1999cl,
and 1999cp we find that the intrinsic V-K colors of Type Ia SNe with
multi-color light curve shape (MLCS) parameter -0.4 < Delta < +0.2 suggest a
uniform color curve. V-K colors become bluer linearly with time from roughly
one week before B-band maximum until one week after maximum, after which they
redden linearly until four weeks after maximum. V-H colors exhibit very similar
color evolution. V-J colors exhibit slightly more complex evolution, with
greater scatter. The existence of V minus near infrared color relations allows
the construction of near infrared light curve templates that are an improvement
on those of Elias et al. (1985).
We provide optical BVRI and infrared JHK photometry of the Type Ia supernovae
1999aa, 1999cl, and 1999cp. SN 1999aa is an overluminous "slow decliner" (with
Delta = -0.47 mag). SN 1999cp is a moderately bright SN unreddened in its host.
SN 1999cl is extremely reddened in its host. The V minus near infrared colors
of SN 1999cl yield A_V = 2.01 +/- 0.11 mag. This leads to a distance for its
host galaxy (M 88) in agreement with other distance measurements for members of
the Virgo cluster.Comment: 57 pages, 13 postscript figures, to appear in the August 20, 2000,
issue of the Astrophysical Journal. Contains updated references and a number
of minor corrections dealt with when page proofs were correcte
Positron Escape from Type Ia Supernovae
We generate bolometric light curves for a variety of type Ia supernova models
at late times, simulating gamma-ray and positron transport for various
assumptions about the magnetic field and ionization of the ejecta. These
calculated light curve shapes are compared with light curves of specific
supernovae for which there have been adequate late observations. %The selection
of models is generally not based upon the %ability to fit the late
observations, but rather because the %model has been demonstrated by other
authors to approximate the spectra %and early light curves of that specific SN.
From these comparisons we draw two conclusions: whether a suggested model is
an acceptable approximation of a particular event, and, given that it is, the
magnetic field characteristics and degree of ionization that are most
consistent with the observed light curve shape. For the ten SNe included in
this study, five strongly suggest Co positron escape as would be
permitted by a weak or radially-combed magnetic field. Of the remaining five
SNe, none clearly show the upturned light curve expected for positron trapping
in a strong, tangled magnetic field. Chandrasekhar mass models can explain
normally, sub-, and super- luminous supernova light curves; sub-Chandrasekhar
mass models have difficulties with sub- (and potentially normally) luminous
SNe. An estimate of the galactic positron production rate from type Ia SNe is
compared with gamma-ray observations of Galactic 511 keV annihilation
radiation. Additionally, we emphasize the importance of correctly treating the
positron transport for calculations of spectra, or any properties, of type Ia
SNe at late epochs ( 200 d).Comment: 82 pages including 25 figure
Cloud fragmentation and proplyd-like features in HII regions imaged by HST
We have analyzed HST ACS and WFPC2 new and archival images of eight HII
regions to look for new proto-planetary disks (proplyds) similar to those found
in the Orion Nebula. We find a wealth of features similar in size (though many
are larger) to the bright cusps around the Orion Nebula proplyds. None of them,
however, contains a definitive central star. From this, we deduce that the new
cusps may not be proplyds, but instead are fragments of molecular cloud
material. Out of all the features found in the eight HII regions examined, only
one, an apparent edge-on silhouette in M17, may have a central star. This
feature might join the small number of bona fide proplyds found outside the
Orion Nebula, in M8, M20 and possibly in M16. In line with the results found
recently by Smith et al. (2005), the paucity of proplyds outside the Orion
Nebula, may be explained by their transient nature as well as by the specific
environmental conditions under whichthey can be observed.Comment: 51 pages; 19 figures; 5 tables. Accepted by A
The Reddening-Free Decline Rate Versus Luminosity Relationship for Type Ia Supernovae
We develop a method for estimating the host galaxy dust extinction for type
Ia supernovae based on an observational coincidence first noted by Lira (1995),
who found that the B-V evolution during the period from 30-90 days after V
maximum is remarkably similar for all events, regardless of light curve shape.
This fact is used to calibrate the dependence of the B(max)-V(max) and
V(max)-I(max) colors on the light curve decline rate parameter delta-m15, which
can, in turn, be used to separately estimate the host galaxy extinction. Using
these methods to eliminate the effects of reddening, we reexamine the
functional form of the decline rate versus luminosity relationship and provide
an updated estimate of the Hubble constant of Ho = 63.3 +- 2.2(internal) +-
3.5(external) km/s/Mpc.Comment: 32 pages, 10 figures, AJ 1999 in pres
Reddening law and interstellar dust properties along Magellanic sight-lines
This study establishes that SMC, LMC and Milky Way extinction curves obey the
same extinction law which depends on the 2200A bump size and one parameter, and
generalizes the Cardelli, Clayton and Mathis (1989) relationship. This suggests
that extinction in all three galaxies is of the same nature. The role of linear
reddening laws over all the visible/UV wavelength range, particularly important
in the SMC but also present in the LMC and in the Milky Way, is also
highlighted and discussed.Comment: accepted for publication in Astrophysics and Space Science. 16 pages,
12 figures. Some figures are colour plot
IAC-Star: a Code for Synthetic Color-Magnitude Diagram Computation
The code IAC-star is presented. It generates synthetic HR and color-magnitude
diagrams (CMDs) and is mainly aimed to star formation history studies in nearby
galaxies. Composite stellar populations are calculated on a star by star basis,
by computing the luminosity, effective temperature and gravity of each star by
direct bi-logarithmic interpolation in the metallicity and age grid of a
library of stellar evolution tracks. Visual (broad band and HST) and infrared
magnitudes are also provided for each star after applying bolometric
corrections. The Padua (Bertelli et al. 1994, Girardi et al. 2000) and Teramo
(Pietrinferni et al. 2004) stellar evolution libraries and various bolometric
corrections libraries are used in the current version. A variety of star
formation rate functions, initial mass functions and chemical enrichment laws
are allowed and binary stars can be computed. Although the main motivation of
the code is the computation of synthetic CMDs, it also provides integrated
masses, luminosities and magnitudes as well as surface brightness fluctuation
luminosities and magnitudes for the total synthetic stellar population, and
therefore it can also be used for population synthesis research. The code is
offered for free use and can be executed at the site {\tt
http://iac-star.iac.es}, with the only requirement of referencing this paper
and crediting as indicated in the site.Comment: Astronomical Journal, in pres
Star Formation Histories from HST Color-Magnitude Diagrams of Six Fields of the LMC
We present results on the analysis of background field stars found in HST
WFPC2 observations of six of the old globular clusters of the Large Magellanic
Cloud. Treated as contaminants by the globular cluster analysts, we produce
V-I,V color-magnitude diagrams of the field stars and use them to explore the
LMC's star formation history. The photometry approaches V~26, well below the
turnoff of an ancient ~14 Gyr LMC population of stars. We use a
maximum-likelihood technique to fit model CMDs drawn from Bertelli et al.
(1994) isochrones to the observed CMDs. We constrain the models by the
age-metallicity relation derived from LMC clusters, test four IMF slopes, and
fit for the reddening, distance modulus, and star formation rate. We find that
we can just resolve structure in SFR(t) with time steps of ~0.15 in log age,
implying a resolution of ~4x10^8 years at an age of 1 Gyr. For a Salpeter IMF,
our derived star formation history for the NGC 1754 field is characterized by
an enhanced star formation rate over the past 4 Gyr, qualitatively resembling
that derived by others for a variety of LMC fields. The remaining four fields,
however, appear to have had high levels of star formation activity as long as
5-8 Gyr ago; these fields lie in the LMC Bar while the NGC 1754 field lies in
the disk, suggesting that the inner regions of the LMC contain significantly
more older stars than the outer regions. Examining the residuals of the models
and observations, we find that the old red giant branches of the models provide
a poor fit to the observations, which suggests an error in the model
isochrones. The effect of the disagreement appears to be to underestimate the
contribution of the old population.Comment: 48 pages, 19 Postscript figures, LaTeX, accepted by the Astronomical
Journa
Stellar Rotation in Young Clusters. I. Evolution of Projected Rotational Velocity Distributions
Open clusters offer us the means to study stellar properties in samples with
well-defined ages and initial chemical composition. Here we present a survey of
projected rotational velocities for a large sample of mainly B-type stars in
young clusters to study the time evolution of the rotational properties of
massive stars. The survey is based upon moderate resolution spectra made with
the WIYN 3.5 m and CTIO 4 m telescopes and Hydra multi-object spectrographs,
and the target stars are members of 19 young open clusters with an age range of
approximately 6 to 73 Myr. We made fits of the observed lines He I 4026, 4387,
4471 and Mg II 4481 using model theoretical profiles to find projected
rotational velocities for a total of 496 OB stars. We find that there are fewer
slow rotators among the cluster B-type stars relative to nearby B stars in the
field. We present evidence consistent with the idea that the more massive B
stars (M > 9 solar masses) spin down during their main sequence phase. However,
we also find that the rotational velocity distribution appears to show an
increase in the numbers of rapid rotators among clusters with ages of 10 Myr
and higher. These rapid rotators appear to be distributed between the zero age
and terminal age main sequence locations in the Hertzsprung-Russell diagram,
and thus only a minority of them can be explained as the result of a spin up at
the terminal age main sequence due to core contraction. We suggest instead that
some of these rapid rotators may have been spun up through mass transfer in
close binary systems.Comment: 33 pages, 11 figures, accepted by Ap
Stellar Rotation in Young Clusters. II. Evolution of Stellar Rotation and Surface Helium Abundance
We derive the effective temperatures and gravities of 461 OB stars in 19
young clusters by fitting the H-gamma profile in their spectra. We use
synthetic model profiles for rotating stars to develop a method to estimate the
polar gravity for these stars, which we argue is a useful indicator of their
evolutionary status. We combine these results with projected rotational
velocity measurements obtained in a previous paper on these same open clusters.
We find that the more massive B-stars experience a spin down as predicted by
the theories for the evolution of rotating stars. Furthermore, we find that the
members of binary stars also experience a marked spin down with advanced
evolutionary state due to tidal interactions. We also derive non-LTE-corrected
helium abundances for most of the sample by fitting the He I 4026, 4387, 4471
lines. A large number of helium peculiar stars are found among cooler stars
with Teff < 23000 K. The analysis of the high mass stars (8.5 solar masses < M
< 16 solar masses) shows that the helium enrichment process progresses through
the main sequence (MS) phase and is greater among the faster rotators. This
discovery supports the theoretical claim that rotationally induced internal
mixing is the main cause of surface chemical anomalies that appear during the
MS phase. The lower mass stars appear to have slower rotation rates among the
low gravity objects, and they have a large proportion of helium peculiar stars.
We suggest that both properties are due to their youth. The low gravity stars
are probably pre-main sequence objects that will spin up as they contract.
These young objects very likely host a remnant magnetic field from their natal
cloud, and these strong fields sculpt out surface regions with unusual chemical
abundances.Comment: 50 pages 18 figures, accepted by Ap
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