42 research outputs found
The pressure confined wind of the massive and compact superstar cluster M82-A1
The observed parameters of the young superstar cluster M82-A1 and its
associated compact HII region are here shown to indicate a low heating
efficiency or immediate loss, through radiative cooling, of a large fraction of
the energy inserted by stellar winds and supernovae during the early evolution
of the cluster. This implies a bimodal hydrodynamic solution which leads to a
reduced mass deposition rate into the ISM, with a much reduced outflow
velocity. Furthermore, to match the observed parameters of the HII region
associated to M82-A1, the resultant star cluster wind is here shown to ought to
be confined by a high pressure interstellar medium. The cluster wind
parameters, as well as the location of the reverse shock, its cooling length
and the radius of the standing outer HII region are derived analytically. All
of these properties are then confirmed with a semi-analytical integration of
the flow equations, which provides us also with the run of the hydrodynamic
variables as a function of radius. The impact of the results is discussed and
extended to other massive and young superstar clusters surrounded by a compact
HII region.Comment: 19 pages, 4 figures, accepted for publication in Ap
Metallicity inhomogeneities in local star-forming galaxies as sign of recent metal-poor gas accretion
We measure the oxygen metallicity of the ionized gas along the major axis of
seven dwarf star-forming galaxies. Two of them, SDSSJ1647+21 and SDSSJ2238+14,
show 0.5 dex metallicity decrements in inner regions with enhanced
star-formation activity. This behavior is similar to the metallicity drop
observed in a number of local tadpole galaxies by Sanchez Almeida et al. (2013)
and interpreted as showing early stages of assembling in disk galaxies, with
the star formation sustained by external metal-poor gas accretion. The
agreement with tadpoles has several implications: (1) it proves that galaxies
other than the local tadpoles present the same unusual metallicity pattern. (2)
Our metallicity inhomogeneities were inferred using the direct method, thus
discarding systematic errors usually attributed to other methods. (3) Taken
together with the tadpole data, our findings suggest a threshold around one
tenth the solar value for the metallicity drops to show up. Although galaxies
with clear metallicity drops are rare, the physical mechanism responsible for
them may sustain a significant part of the star-formation activity in the local
Universe. We argue that the star-formation dependence of the mass-metallicity
relationship, as well as other general properties followed by most local disk
galaxies, are naturally interpreted as side effects of pristine gas infall.
Alternatives to the metal poor gas accretion are examined too.Comment: Accepted for publication in ApJ. 10 pages. 5 Fig