67 research outputs found
HST Imaging of Globular Clusters in the Edge--on Spiral Galaxies NGC 4565 and NGC 5907
We present a study of the globular cluster systems of two edge-on spiral
galaxies, NGC4565 and NGC5907, from WFPC2 images in the F450W and F814W
filters. The globular cluster systems of both galaxies appear to be similar to
the Galactic globular cluster system. In particular, we derive total numbers of
globular clusters of N_{GC}(4565)= 204+/-38 {+87}{-53} and
N_{GC}(5907)=170+/-41 {+47}{-72} (where the first are statistical, the second
potential systematic errors) for NGC4565 and NGC5907, respectively. This
determination is based on a comparison to the Milky Way system, for which we
adopt a total number of globular clusters of 180+/-20. The specific frequency
of both galaxies is S_N~0.6: indistinguishable from the value for the Milky
Way. The similarity in the globular cluster systems of the two galaxies is
noteworthy since they have significantly different thick disks and
bulge-to-disk ratios. This would suggest that these two components do not play
a major role in the building up of a globular cluster system around late-type
galaxies.Comment: Accepted for publication in the Astronomical Journal, 17 pages incl 5
figures, AAS style two columns. Also available at
http://www.eso.org/~mkissler, Color version of figure 1 only available at
http://www.eso.org/~mkissler (B/W version included
On the Progenitors of Two Type II-P Supernovae in the Virgo Cluster
Direct identification of the progenitors of supernovae (SNe) is rare because
of the required spatial resolution and depth of the archival data prior to the
SN explosions. Here we report on the identification of the progenitors of two
nearby SNe in the Virgo cluster: SN 2006my in NGC 4651 and SN 2006ov in M61. We
obtained high-quality ground-based images of SN 2006my with the
Canada-France-Hawaii Telescope, and are able to locate the site of the SN on
pre-SN {\it Hubble Space Telescope} ({\it HST}) Wide Field Planetary Camera 2
images to a high precision (1 uncertainty of \pm0\farcs05). We
pinpoint the site of SN 2006ov to within 0\farcs02 from {\it HST} Advanced
Camera for Surveys images of the SN. We detected a red supergiant progenitor
for each SN within the error circles, with an inferred zero-age main-sequence
mass () of and
for the progenitors of SNe 2006my and 2006ov, respectively. The mass estimates
for the progenitors of both SNe confirm a suggested trend that the most common
Type II-plateau SNe originate from low-mass supergiants with --20 .Comment: Accepted version with numerous minor change
On the Progenitor of the Type II Supernova 2004et in NGC 6946
Supernova (SN) 2004et is the eighth historical SN in the nearby spiral galaxy
NGC 6946. Here we report on early photometric and spectroscopic monitoring of
this object. SN 2004et is a Type II event, exhibiting a plateau in its light
curves, but its spectral and color evolution appear to differ significantly
from those of other, more normal Type II-plateau (II-P) SNe. We have analyzed
Canada-France-Hawaii Telescope (CFHT) images of the host galaxy taken prior to
the SN explosion, identifying a candidate progenitor for the SN. The star's
absolute magnitude and intrinsic color imply that it was a yellow, rather than
red, supergiant star, with an estimated zero-age main sequence mass of
. Although this mass estimate is consistent with
estimates and upper limits for the progenitors of other, more normal SNe II-P,
the SN 2004et progenitor's unusual color could further imply a pre-explosion
evolutionary history analogous to, but less extreme than, that for the
progenitors of the peculiar Type II-P SN 1987A or the Type IIb SN 1993J. The
identity of the progenitor candidate needs to be verified when the SN has
significantly dimmed.Comment: To appear in PASP (Feb 2005). A high resolution PostScript version is
available at http://astron.berkeley.edu/~weidong/ms_04et.ps.g
A multivariate statistical analysis of spiral galaxy luminosities. I. Data and results
We have performed a multiparametric analysis of luminosity data for a sample
of 234 normal spiral and irregular galaxies observed in X-rays with the {\it
Einstein Observatory}. This sample is representative of S and Irr galaxies,
with a good coverage of morphological types and absolute magnitudes. In
addition to X-ray and optical da ta, we have compiled H-band magnitudes, IRAS
near- and far-infrared, and 6cm radio co ntinuum observations for the sample
from the literature. We have also performed a carefu l compilation of distance
estimates. We have explored the effect of morphology by dividing the sample
into early (S0/a-Sab), intermediate (Sb-Sbc), and late-type (Sc-Irr)
subsamples. The data were analysed with bivariate and multiv ariate survival
analysis techniques that make full use of all the information available in both
detections and limits
SN 2008S: an electron capture SN from a super-AGB progenitor?
We present comprehensive photometric and spectroscopic observations of the
faint transient SN 2008S discovered in NGC 6946. SN 2008S exhibited slow
photometric evolution and almost no spectral variability during the first nine
months, implying a high density CS medium. The light curve is similar in shape
to that of SN 1998S and SN 1979C, although significantly fainter at maximum
light. Our quasi-bolometric lightcurve extends to 300 days and shows a tail
phase decay rate consistent with that of ^{56}Co. We propose that this is
evidence for an explosion and formation of ^{56}Ni (0.0015 +/- 0.0004 M_Sun).
The large MIR flux detected shortly after explosion can be explained by a light
echo from pre-exisiting dust. The late NIR flux excess is plausibly due to a
combination of warm newly-formed ejecta dust together with shock-heated dust in
the CS environment. We reassess the progenitor object detected previously in
Spitzer archive images, supplementing this discussion with a model of the MIR
spectral energy distribution. This supports the idea of a dusty, optically
thick shell around SN 2008S with an inner radius of nearly 90AU and outer
radius of 450AU, and an inferred heating source of 3000 K and luminosity of L ~
10^{4.6} L_Sun. The combination of our monitoring data and the evidence from
the progenitor analysis leads us to support the scenario of a weak electron
capture supernova explosion in a super-AGB progenitor star (of initial mass 6-8
M_sun) embedded within a thick CS gaseous envelope. We suggest that all of main
properties of the electron capture SN phenomenon are observed in SN 2008S and
future observations may allow a definitive answer.Comment: accepted for publication in MNRAS (2009 May 7
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