127 research outputs found
The B[e] phenomenon in the Milky Way and Magellanic Clouds
Discovered over 30 years ago, the B[e] phenomenon has not yet revealed all
its puzzles. New objects that exhibit it are being discovered in the Milky Way,
and properties of known objects are being constrained. We review recent
findings about objects of this class and their subgroups as well as discuss new
results from studies of the objects with yet unknown nature. In the Magellanic
Clouds, the population of such objects has been restricted to supergiants. We
present new candidates with apparently lower luminosities found in the LMC.Comment: 5 pages, 2 figures, IAU Symposium 272, Active OB stars: structure,
evolution, mass loss and critical limit
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
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
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
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
The extinction curve in the visible and the value of Rv
This article discusses the interstellar extinction curve in the visible and
the value of Rv. It is concluded that the visible extinction curve is likely to
be linear in the visible, and that indirect estimates of Rv from tentative
determinations of Av, infrared, or UV observations are questionable. There is
currently no evidence of any variation of Rv with direction. If Rv is close to
3, as it has been inferred from mid-infrared data, starlight in the visible is
extinguished by a factor F/F_0=(2.5exp{-2micron/lambda})^{E(B-V)} in the
visible. But if the visible wavelength range alone is considered, 4 appears as
its most natural and probable value, and F/F_0= exp{-2E(B-V)/lambda}.Comment: 4 figures. Published in Astronomische Nachrichten. Label of Fig. 1 is
corrected in the present version. Paper previously rejected by MNRAS (MJ
Barlow editor, review is available
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
Spectroscopic study of the N159/N160 complex in the Large Magellanic Cloud
We present a spectroscopic study of the N159/N160 massive-star forming region
south of 30 Doradus in the Large Magellanic Cloud, classifying a total of 189
stars in the field of the complex. Most of them belong to O and early B
spectral classes; we have also found some uncommon and very interesting
spectra, including members of the Onfp class, a Be P Cygni star, and some
possible multiple systems. Using spectral types as broad indicators of
evolutionary stages, we considered the evolutionary status of the region as a
whole. We infer that massive stars at different evolutionary stages are present
throughout the region, favoring the idea of a common time for the origin of
recent star formation in the N159/N160 complex as a whole, while sequential
star formation at different rates is probably present in several subregions.Comment: 36 pages, 24 figures (127 spectra mostly OB stars, 4 field images).
Published in The Astronomical Journa
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
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