952 research outputs found
Discovery of a massive variable star with Z=Zo/36 in the galaxy DDO 68
The Local Volume dwarf galaxy DDO 68, from the spectroscopy of its two
brightest HII regions (Knots 1 and 2) was designated as the second most
metal-poor star-forming galaxy [12+log(O/H)=7.14]. In the repeated spectral
observations in 2008 January with the 6-m telescope (BTA) of the HII region
Knot 3 [having 12+log(O/H)=7.10+-0.06], we find a strong evidence of a
transient event related to a massive star evolution. From the follow-up
observation with the higher spectral resolution in 2008 February, we confirm
this phenomenon, and give parameters of its emission-line spectrum comprising
of Balmer HI and HeI lines. The luminosities of the strongest transient lines
(Ha, Hb) are of a few 10^36 erg s^-1. We also detected an additional continuum
component in the new spectrum of Knot 3, which displays the spectral energy
distribution raising to ultraviolet. The estimate of the flux of this continuum
leads us to its absolute V-band magnitude of ~-7.1. Based on the spectral
properties of this transient component, we suggest that it is related to an
evolved massive star of luminous blue variable type with Z=Zo/36. We briefly
discuss observational constraints on parameters of this unique (in the aspect
of the record low metallicity of the progenitor massive star) event and propose
several lines of its study.Comment: 6 pages, 5 Postscript figures, to appear in MNRAS Letters in June
2008 issu
VLT/GIRAFFE spectroscopic observations of the metal-poor blue compact dwarf galaxy SBS 0335-052E
We present two-dimensional spectroscopy of the extremely metal-deficient blue
compact dwarf (BCD) galaxy SBS 0335-052E aiming to studyphysical conditions,
element abundances and kinematical properties of the ionised gas in this
galaxy. Observations were obtained in the spectral range 3620-9400A with the
imaging spectrograph GIRAFFE installed on the UT2 of the Very Large Telescope
(VLT). These observations are the first ones carried out so far with GIRAFFE in
the ARGUS mode which allows to obtain simultaneously 308 spectra covering a
11.4"x7.3" region. We produced images of SBS 0335-052E in the continuum and in
emission lines of different stages of excitation. We find that while the
maximum of emission in the majority of lines, including the strong lines Hbeta
4861A, Halpha 6563A, [OIII] 4363,5007A, [OII] 3726,3729A, coincides with the
youngest south-eastern star clusters 1 and 2, the emission of HeII 4686A line
is offset to the more evolved north-west clusters 4,5. This suggests that hard
ionising radiation responsible for the HeII 4686A emission is not related to
the most massive youngest stars, but rather is connected with fast radiative
shocks. This conclusion is supported by the kinematical properties of the
ionised gas from the different emission lines as the velocity dispersion in the
HeII 4686A line is systematically higher, by ~50%-100%, than that in other
lines. The variations of the emission line profiles suggest the presence of an
ionised gas outflow in the direction perpendicular to the galaxy disk.
(abridged)Comment: 21 pages, 11 figures, accepted for publication in Astronomy and
Astrophysic
A New Approach to Systematic Uncertainties and Self-Consistency in Helium Abundance Determinations
Tests of big bang nucleosynthesis and early universe cosmology require
precision measurements for helium abundance determinations. However, efforts to
determine the primordial helium abundance via observations of metal poor H II
regions have been limited by significant uncertainties. This work builds upon
previous work by providing an updated and extended program in evaluating these
uncertainties. Procedural consistency is achieved by integrating the hydrogen
based reddening correction with the helium based abundance calculation, i.e.,
all physical parameters are solved for simultaneously. We include new atomic
data for helium recombination and collisional emission based upon recent work
by Porter et al. and wavelength dependent corrections to underlying absorption
are investigated. The set of physical parameters has been expanded here to
include the effects of neutral hydrogen collisional emission. Because of a
degeneracy between the solutions for density and temperature, the precision of
the helium abundance determinations is limited. Also, at lower temperatures (T
\lesssim 13,000 K) the neutral hydrogen fraction is poorly constrained
resulting in a larger uncertainty in the helium abundances. Thus the derived
errors on the helium abundances for individual objects are larger than those
typical of previous studies. The updated emissivities and neutral hydrogen
correction generally raise the abundance. From a regression to zero
metallicity, we find Y_p as 0.2561 \pm 0.0108, in broad agreement with the WMAP
result. Tests with synthetic data show a potential for distinct improvement,
via removal of underlying absorption, using higher resolution spectra. A small
bias in the abundance determination can be reduced significantly and the
calculated helium abundance error can be reduced by \sim 25%.Comment: 51 pages, 13 figure
Gas metallicity diagnostics in star-forming galaxies
Generally the gas metallicity in distant galaxies can only be inferred by
using a few prominent emission lines. Various theoretical models have been used
to predict the relationship between emission line fluxes and metallicity,
suggesting that some line ratios can be used as diagnostics of the gas
metallicity in galaxies. However, accurate empirical calibrations of these
emission line flux ratios from real galaxy spectra spanning a wide metallicity
range are still lacking. In this paper we provide such empirical calibrations
by using the combination of two sets of spectroscopic data: one consisting of
low-metallicity galaxies with a measurement of [OIII]4363, and the other one
consisting of galaxies in the SDSS DR4 whose metallicity has been determined
from various strong emission lines in their spectra. This combined data set
constitutes the largest sample of galaxies with information on the gas
metallicity available so far and spanning the widest metallicity range. Our
empirical diagrams show that the line ratio [OIII]/[OII] is a useful tool to
break the degeneracy in the R_23 parameter when no information on the [NII]
line is available. The line ratio [NeIII]/[OII] also results to be a useful
metallicity indicator for high-z galaxies. Finally, we compare these empirical
relations with photoionization models. We find that the empirical
R_23-metallicity sequence is strongly discrepant with respect to the trend
expected by models with constant ionization parameter, which is interpreted as
a consequence of a strong metallicity dependence of the average ionization
parameter in galaxies. This result should warn about the use of theoretical
models with constant ionization parameter to infer metallicities from observed
line ratios. (abbreviated)Comment: 27 pages, 19 figures, Astronomy and Astrophysics, in press. (Replaced
with the accepted version.
The metallicities of UM151, UM408 and A1228+12 revisited
We present the results of new spectrophotometry and heavy element abundance
determinations for 3 dwarf galaxies UM151, UM408 and A1228+12 (RMB132). These
galaxies have been claimed in the literature to have very low metallicities,
corresponding to log(O/H)+12 < 7.65, that are in the metallicity range of some
candidate local young galaxies. We present higher S/N data for these three
galaxies. UM151 and UM408 have significantly larger metallicities: log(O/H)+12
= 8.5 and 7.93, respectively. For A1228+12 our new log(O/H)+12 = 7.73 is close
to that recalculated from earlier data (7.68). Thus, the rederived
metallicities allow us to remove these objects from the list of galaxies with Z
< 1/20 Z_Sun.Comment: LaTeX, 8 pages with 3 Postscript figures, A&A in pres
On ionisation effects and abundance ratios in damped Lyman-alpha systems
The similarity between observed velocity structures of Al III and singly
ionised species in damped Lyman-alpha systems (DLAs) suggests the presence of
ionised gas in the regions where most metal absorption lines are formed.
To explore the possible implications of ionisation effects we construct a
simplified two-region model for DLAs consisting of an ionisation bounded region
with an internal radiation field and a neutral region with a lower metal
content. Within this framework we find that ionisation effects are important.
If taken into account, the element abundance ratios in DLAs are quite
consistent with those observed in Milky Way stars and in metal-poor H II
regions in blue compact dwarf galaxies. In particular we cannot exclude the
same primary N origin in both DLAs and metal-poor galaxies. From our models no
dust depletion of heavy elements needs to be invoked; little depletion is
however not excluded.Comment: to appear in "Evolution of Galaxies. I. Observational clues", Eds.
J.M. Vilchez, G. Stasinska, Astrophysics and Space Science, in press. 5
pages, including 3 figure
HST observations of the blue compact dwarf SBS 0335-052: a probable young galaxy
We present HST WFPC2 V and I images and GHRS UV spectrophotometry of the
spectral regions around Ly and OI 1302 of the extremely metal-deficient
(Z~Zsun/41) blue compact dwarf (BCD) galaxy SBS 0335-052. All the star
formation in the BCD occurs in six super-star clusters (SSC) with ages =< 3-4
Myr. Dust is clearly present and mixed spatially with the SSCs. There is a
supershell of radius ~380 pc, delineating a large supernova cavity. The
instantaneous star formation rate is ~0.4 Msun yr^-1. Strong narrow Ly
emission is not observed. Rather there is low intensity broad (FWZI = 20 A)
Ly emission superposed on even broader Ly absorption by the HI
envelope. This broad low-intensity emission is probably caused by resonant
scattering of Ly photons. The BCD appears to be a young galaxy,
undergoing its very first burst of star formation. This conclusion is based on
the following evidence: 1) the underlying extended low-surface-brightness
component is very irregular and filamentary, suggesting that a significant part
of the emission comes from ionized gas; 2) it has very blue colors (-0.34 =<
(V-I) =< 0.16), consistent with gaseous emission colors; 3) the OI 1302
line is not detected in absorption in the GHRS spectrum, setting an upper limit
for N(O)/N(H) in the HI envelope of the BCD of more than 3000 times smaller
than the value in Orion.Comment: 20 pages and 6 Postscript figures. Submitted to Astrophysical Journa
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