356 research outputs found
Mean age gradient and asymmetry in the star formation history of the Small Magellanic Cloud
We derive the star formation history in four regions of the Small Magellanic
Cloud (SMC) using the deepest VI color-magnitude diagrams (CMDs) ever obtained
for this galaxy. The images were obtained with the Advanced Camera for Surveys
onboard the Hubble Space Telescope and are located at projected distances of
0.5-2 degrees from the SMC center, probing the main body and the wing of the
galaxy. We derived the star-formation histories (SFH) of the four fields using
two independent procedures to fit synthetic CMDs to the data. We compare the
SFHs derived here with our earlier results for the SMC bar to create a deep
pencil-beam survey of the global history of the central SMC. We find in all the
six fields observed with HST a slow star formation pace from 13 to 5-7 Gyr ago,
followed by a ~ 2-3 times higher activity. This is remarkable because dynamical
models do not predict a strong influence of either the LMC or the Milky Way
(MW) at that time. The level of the intermediate-age SFR enhancement
systematically increases towards the center, resulting in a gradient in the
mean age of the population, with the bar fields being systematically younger
than the outer ones. Star formation over the most recent 500 Myr is strongly
concentrated in the bar, the only exception being the area of the SMC wing. The
strong current activity of the latter is likely driven by interaction with the
LMC. At a given age, there is no significant difference in metallicity between
the inner and outer fields, implying that metals are well mixed throughout the
SMC. The age-metallicity relations we infer from our best fitting models are
monotonically increasing with time, with no evidence of dips. This may argue
against the major merger scenario proposed by Tsujimoto and Bekki 2009,
although a minor merger cannot be ruled out.Comment: 30 pages, 16 figures, accepted for publication in Ap
A High-Resolution Multiband Survey of Westerlund 2 With the Hubble Space Telescope I: Is the Massive Star Cluster Double?
We present first results from a high resolution multi-band survey of the
Westerlund 2 region with the Hubble Space Telescope. Specifically, we imaged
Westerlund 2 with the Advanced Camera for Surveys through the , ,
and filters and with the Wide Field Camera 3 in the , ,
and filters. We derive the first high resolution pixel-to-pixel map of
the color excess of the gas associated with the cluster, combining
the H () and Pa () line observations. We
demonstrate that, as expected, the region is affected by significant
differential reddening with a median of ~mag. After separating
the populations of cluster members and foreground contaminants using a
vs. color-magnitude diagram, we identify a pronounced
pre-main-sequence population in Westerlund 2 showing a distinct turn-on. After
dereddening each star of Westerlund 2 individually in the color-magnitude
diagram we find via over-plotting PARSEC isochrones that the distance is in
good agreement with the literature value of ~kpc. With
zero-age-main-sequence fitting to two-color-diagrams, we derive a value of
total to selective extinction of . A spatial density map of
the stellar content reveals that the cluster might be composed of two clumps.
We estimate the same age of 0.5-2.0 Myr for both clumps. While the two clumps
appear to be coeval, the northern clump shows a lower stellar
surface density.Comment: 24 pages, 27 figures, 7 tables; Accepted for publication to The
Astronomical Journa
On the Structure and Kinematics of Nebulae around LBVs and LBV Candidates in the LMC
We present a detailed analysis of the morphology and kinematics of nebulae
around LBVs and LBV candidates in the Large Magellanic Cloud. HST images and
high-resolution Echelle Spectra were used to determine the size, shape,
brightness, and expansion velocities of the LBV nebulae around R127, R143, and
S61. For S Dor, R71, R99, and R84 we discuss the possible presence of nebular
emission, and derive upper limits for the size and lower limits on the
expansion velocities of possible nebulae. Including earlier results for the LBV
candidates S119 and SK-69 279 we find that in general the nebulae around LBVs
in the LMC are comparable in size to those found in the Milky Way. The
expansion velocities of the LMC nebulae, however, are significantly lower--by
about a factor of 3 to 4--than those of galactic nebulae of comparable size.
Galactic and LMC nebulae show about the same diversity of morphologies, but
only in the LMC do we find nebulae with outflow. Bipolarity--at least to some
degree--is found in nebulae in the LMC as well as in the Milky Way, and
manifests a much more general feature among LBV nebulae than previously known.Comment: paper accepted by A&A, 25 pages, 24 figures; paper with images in
full resolution available at
http://www.astro.ruhr-uni-bochum.de/kweis/publications.htm
The massive star population of the Virgo Cluster galaxy NGC 4535
We analyzed the massive star population of the Virgo Cluster galaxy NGC 4535
using archival Hubble Space Telescope Wide Field Planetary Camera 2 images in
filters F555W and F814W, equivalent to Johnson V and Kron-Cousins I. We
performed high precision point spread function fitting photometry of 24353
sources including 3762 candidate blue supergiants, 841 candidate yellow
supergiants and 370 candidate red supergiants. We estimated the ratio of blue
to red supergiants as a decreasing function of galactocentric radius. Using
Modules for Experiments in Stellar Astrophysics isochrones at solar
metallicity, we defined the luminosity function and estimated the star
formation history of the galaxy over the last 60 Myrs. We conducted a
variability search in the V and I filters using three variability indexes: the
median absolute deviation, the interquartile range and the inverse von-Neumann
ratio. This analysis yielded 120 new variable candidates with absolute
magnitudes ranging from M = 4 to 11 mag. We used the MESA
evolutionary tracks at solar metallicity, to classify the variables based on
their absolute magnitude and their position on the color-magnitude diagram.
Among the new candidate variable sources are eight candidate variable red
supergiants, three candidate variable yellow supergiants and one candidate
luminous blue variable, which we suggest for follow-up observations.Comment: Accepted by A&A, 7 pages, 7 Tables, 53 figure
An outflow from the nebula around the LBV candidate S 119
We present an analysis of the kinematic and morphological structure of the
nebula around the LMC LBV candidate S 119. On HST images, we find a
predominantly spherical nebula which, however, seems to be much better confined
in its eastern hemisphere than in the western one. The filamentary western part
of the nebula is indicative of matter flowing out of the nebula's main body.
This outflow is even more evidenced by our long-slit echelle spectra. They show
that, while most of the nebula has an expansion velocity of 25.5 km/s, the
outflowing material reaches velocities of almost 140 km/s, relative to the
systemic one. A ROSAT HRI image shows no trace of S 119 and thus no indications
of hot or shocked material.Comment: 8 pages, 8 figures, Accepted in A&A, paper with images in full
resolution at http://www.mpifr-bonn.mpg.de/staff/kweis/publications.htm
Star Formation History in two fields of the Small Magellanic Cloud Bar
The Bar is the most productive region of the Small Magellanic Cloud in terms
of star formation but also the least studied one. In this paper we investigate
the star formation history of two fields located in the SW and in the NE
portion of the Bar using two independent and well tested procedures applied to
the color-magnitude diagrams of their stellar populations resolved by means of
deep HST photometry. We find that the Bar experienced a negligible star
formation activity in the first few Gyr, followed by a dramatic enhancement
from 6 to 4 Gyr ago and a nearly constant activity since then. The two examined
fields differ both in the rate of star formation and in the ratio of recent
over past activity, but share the very low level of initial activity and its
sudden increase around 5 Gyr ago. The striking similarity between the timing of
the enhancement and the timing of the major episode in the Large Magellanic
Cloud is suggestive of a close encounter triggering star formation.Comment: 30 pages, 22 figures, accepted for publication in Ap
SN2002kg -- the brightening of LBV V37 in NGC 2403
SN2002kg is a type IIn supernova, detected in October 2002 in the nearby
spiral galaxy NGC 2403. We show that the position of SN2002kg agrees within the
errors with the position of the LBV V37. Ground based and HST ACS images
however show that V37 is still present after the SN2002kg event. We compiled a
lightcurve of V37 which underlines the variablity of the object, and shows that
SN2002kg was the brightening of V37 and not a supernova. The recent brightening
is not a giant eruption, but more likely part of an S Dor phase. V37 shows
strong Halpha +[NII] emission in recent images and in the SN2002kg spectrum,
which we interprete as the signature of the presence of an LBV nebula. A
historic spectrum lacks emission, which may hint that we are witnessing the
formation of an LBV nebula.Comment: 4 pages, 5 figures, accepted in A&A Letter, paper with images in full
resolution at http://www.astro.ruhr-uni-bochum.de/kweis/publications.htm
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