280 research outputs found
New Wolf-Rayet star and its circumstellar nebula in Aquila
We report the discovery of a new Wolf-Rayet star in Aquila via detection of
its circumstellar nebula (reminiscent of ring nebulae associated with late WN
stars) using the Spitzer Space Telescope archival data. Our spectroscopic
follow-up of the central point source associated with the nebula showed that it
is a WN7h star (we named it WR121b). We analyzed the spectrum of WR121b by
using the Potsdam Wolf-Rayet (PoWR) model atmospheres, obtaining a stellar
temperature of ~ 50 kK. The stellar wind composition is dominated by helium
with ~ 20 per cent of hydrogen. The stellar spectrum is highly reddened
(E_{B-V} = 2.85 mag). Adopting an absolute magnitude of M_v = -5.7, the star
has a luminosity of log L/Lsun = 5.75 and a mass-loss rate of 10^{-4.7}
Msun/yr, and resides in a distance of 6.3 kpc. We searched for a possible
parent cluster of WR121b and found that this star is located at ~ 1 degree from
the young star cluster embedded in the giant HII region W43 (containing a
WN7+a/OB? star -- WR121a). We also discovered a bow shock around the O9.5III
star ALS9956, located at ~ 0.5 degree from the cluster. We discuss the
possibility that WR121b and ALS9956 are runaway stars ejected from the cluster
in W43.Comment: 9 pages, 7 figures, accepted to MNRA
IC10: the history of the nearest starburst galaxy through its Planetary Nebula and HII region populations
We report the results of spectroscopic observations, obtained with the Gemini
North Multi-Object Spectrograph, of 9 planetary nebulae (PNe) and 15 \hii\
regions located in the 5.5\arcmin 5.5\arcmin inner region of the nearby
starburst galaxy IC10. Twelve new candidate PNe have been discovered during our
pre-imaging phase. Nine of them have been spectroscopically confirmed. The
direct availability of the electron temperature diagnostics in several nebulae
allowed an accurate determination of the metallicity map of IC10 at two epochs:
the present-time from \hii regions and the old/intermediate-age from PNe. We
found a non-homogeneous distribution of metals at both epochs, but similar
average abundances were found for the two populations. The derived
age-metallicity relation shows a little global enrichment interpreted as the
loss of metals by SN winds and to differential gas outflows. Finally, we
analyzed the production of oxygen --through the third dredge-up-- in the
chemical abundance patterns of the PN populations belonging to several dwarf
irregular galaxies. We found that the third dredge-up of oxygen is a
metallicity dependent phenomenon occurring mainly for 12+(O/H)7.7
and substantially absent in IC10 PNe.Comment: 14 pages, 4 figures, 7 tables, accepted for publication by MNRA
Extragalactic Planetary Nebulae: tracers of the chemical evolution of nearby galaxies
The study of the chemical composition of Planetary Nebulae in external
galaxies is of paramount importance in the fields of stellar evolution and of
the chemical enrichment history of galaxies. In the last years a number of
spectroscopic studies with 6-8m-class telescopes have been devoted to this
subject improving our knowledge of, among other, the time-evolution of the
radial metallicity gradient in disk galaxies, the chemical evolution of dwarf
galaxies, and the stellar evolution at low metallicity.Comment: 8 pages, 4 figures, Invited Review to IAU Symposium 283, "Planetary
Nebulae: an Eye to the Future", Tenerife, 25-29 July 201
Hubble Space Telescope Imaging of the Ultracompact Blue Dwarf Galaxy HS 0822+3542: An Assembling Galaxy in a Local Void?
We present deep U, narrow-V, and I-band images of the ultracompact blue dwarf
galaxy HS 0822+3542, obtained with the Advanced Camera for Surveys / High
Resolution Channel of the Hubble Space Telescope. This object is extremely
metal-poor (12 + log(O/H) = 7.45) and resides in a nearby void. The images
resolve it into two physically separate components that were previously
described as star clusters in a single galaxy. The primary component is only
\~100 pc in maximum extent, and consists of starburst region surrounded by a
ring-like structure of relatively redder stars. The secondary component is ~50
pc in size and lies at a projected distance of ~80 pc away from the primary,
and is also actively star-forming. We estimate masses ~10^7 M(sol) and ~10^6
M(sol) for the two components, based on their luminosities, with an associated
dynamical timescale for the system of a few Myr. This timescale and the
structure of the components suggests that a collision between them triggered
their starbursts. The spectral energy distributions of both components can be
fitted by the combination of recent (few Myr old) starburst and an evolved
(several Gyr old) underlying stellar population, similar to larger blue compact
dwarf galaxies. This indicates that despite its metal deficiency the object is
not forming its first generation of stars. However, the small sizes and masses
of the two components suggests that HS 0822+3542 represents a dwarf galaxy in
the process of assembling from clumps of stars intermediate in size between
globular clusters and objects previously classified as galaxies. Its relatively
high ratio of neutral gas mass to stellar mass (~1) and high specific star
formation rate, log(SFR/M(sol) = -9.2, suggests that it is still converting
much of its gas to stars.Comment: 11 pages, 2 figures, accepted for publication in Astrophysical
Journal Letter
Mid Infrared Properties of Low Metallicity Blue Compact Dwarf Galaxies From Spitzer/IRS
We present a {\em Spitzer}-based mid-infrared study of a large sample of Blue
Compact Dwarf galaxies (BCD) using the Infrared Spectrograph (IRS), including
the first mid-IR spectrum of IZw18, the archetype for the BCD class and among
the most metal poor galaxies known. We show the spectra of Polycyclic Aromatic
Hydrocarbon (PAH) emission in low-metallicity environment. We find that the
equivalent widths (EW) of PAHs at 6.2, 7.7, 8.6 and 11.2 m are generally
weaker in BCDs than in typical starburst galaxies and that the fine structure
line ratio, [NeIII]/[NeII], has a weak anti-correlation with the PAH EW. A much
stronger anti-correlation is shown between the PAH EW and the product of the
[NeIII]/[NeII] ratio and the UV luminosity density divided by the metallicity.
We conclude that PAH EW in metal-poor high-excitation environments is
determined by a combination of PAH formation and destruction effects.Comment: 41 pages, 14 figure
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