247 research outputs found
Age Dating Stellar Populations in the Near Infrared: An absolute age indicator from the presence/absence of red supergiants
The determination of age is a critical component in the study of a population
of stellar clusters. In this letter we present a new absolute age indicator for
young massive star clusters based on J-H colour. This novel method identifies
clusters as older or younger than 5.7 +/- 0.8 Myr based on the appearance of
the first population of red supergiant stars. We test the technique on the
stellar cluster population of the nearby spiral galaxy, M83, finding good
agreement with the theoretical predictions. The localisation of this technique
to the near-IR promises that it may be used well into the future with space--
and ground--based missions optimised for near-IR observations.Comment: 5 pages, 5 figures. Accepted to MNRAS, November 201
Extragalactic Stellar Astronomy with the Brightest Stars in the Universe
A supergiants are objects in transition from the blue to the red (and vice
versa) in the uppermost HRD. They are the intrinsically brightest "normal"
stars at visual light with absolute visual magnitudes up to -9. They are ideal
to study young stellar populations in galaxies beyond the Local Group to
determine chemical composition and evolution, interstellar extinction,
reddening laws and distances. We discuss most recent results on the
quantitative spectral analysis of such objects in galaxies beyond the Local
Group based on medium and low resolution spectra obtained with the ESO VLT and
Keck. We describe the analysis method including the determination of
metallicity and metallicity gradients. A new method to measure accurate
extragalactic distances based on the stellar gravities and effective
temperatures is presented, the flux weighted gravity - luminosity relationship
(FGLR). The FGLR is a purely spectroscopic method, which overcomes the
untertainties introduced by interstellar extinction and variations of
metallicity, which plague all photometric stellar distance determination
methods. We discuss the perspectives of future work using the giant
ground-based telescopes of the next generation such as the TMT, the GMT and the
E-ELT.Comment: Invited review; to appear in "Massive Stars as Cosmic Engines", IAU
Symp. 250, ed. F. Bresolin, P. A. Crowther, and J. Puls (Cambridge University
Press
Stellar spectroscopy far beyond the Local Group
Multi-object spectroscopic observations of blue supergiants in NGC 3621, a
spiral galaxy at a distance of 6.7 Mpc, carried out with the ESO VLT and FORS
are presented. We demonstrate the feasibility of quantitative stellar
spectroscopy at distances approaching a ten-fold increase over previous
investigations by determining chemical composition, stellar parameters,
reddening, extinction and wind properties of one of our targets, a supergiant
of spectral type A1 Ia located in the outskirts of NGC 3621. The metallicity
(determined from iron group elements) is reduced by a factor of two relative to
the sun in qualitative agreement with results from previous abundance studies
based on H II region oxygen emission lines. Reddening and extinction are E(B-V)
= 0.12 and Av = 0.37, respectively, mostly caused by the galactic foreground.
Comparing stellar wind momentum and absolute V magnitude with galactic and M31
counterparts we confirm the potential of the wind momentum-luminosity
relationship as an alternative tool to estimate extragalactic distances.Comment: 9 pages, 4 figures, 1 table, accepted for publication in The
Astrophysical Journal Letter
The Physical Properties and Effective Temperature Scale of O-type Stars as a Function of Metallicity. I. A Sample of 20 Stars in the Magellanic Clouds
We have obtained HST and ground-based observations of a sample of 20 O-type
stars in the LMC and SMC, including six of the hottest massive stars known
(subtypes O2-3) in the R136 cluster. In general, these data include (a) the HST
UV spectra in order to measure the terminal velocities of the stellar winds,
(b) high signal-to-noise, blue-optical data where the primary temperature- and
gravity-sensitive photospheric lines are found, and (c) nebular-free H-alpha
profiles, which provide the mass-loss rates. The line-blanketed non-LTE
atmosphere code FASTWIND was then used to determine the physical parameters of
this sample of stars. We find good agreement between the synthetic line
profiles for the hydrogen, He I, and He II lines in the majority of the stars
we analyzed; the three exceptions show evidence of being incipiently resolved
spectroscopic binaries or otherwise spectral composites. One such system is
apparently an O3 V+O3 V eclipsing binary, and a follow-up radial velocity study
is planned to obtain Keplerian masses. Although we did not use them to
constrain the fits, good agreement is also found for the He I
and He II lines in the near-UV, which we plan to exploit in
future studies. Our effective temperatures are compared to those recently
obtained by Repolust, Puls & Herrero for a sample of Galactic stars using the
same techniques. We find that the Magellanic Cloud sample is
3,000-4,000K hotter than their Galactic counterparts for the early
through mid-O's. These higher temperatures are the consequence of a decreased
importance of wind emission, wind blanketing, and metal-line blanketing at
lower metallicities.Comment: Accepted for publication in the Astrophysical Journal. A postscript
version with the figures embedded can be found at
ftp://ftp.lowell.edu/pub/massey/haw.p
Spectral Modelling of Star-Forming Regions in the Ultraviolet: Stellar Metallicity Diagnostics for High Redshift Galaxies
The chemical composition of high redshift galaxies is an important property
which gives clues to their past history and future evolution and yet is
difficult to measure with current techniques. In this paper we investigate new
metallicity indicators, based upon the strengths of stellar photospheric
features at rest-frame ultraviolet wavelengths. By combining the evolutionary
spectral synthesis code Starburst99 with the output from the non-LTE model
atmosphere code WM-basic, we have developed a code that can model the
integrated ultraviolet stellar spectra of star-forming regions at metallicities
between 1/20 and twice solar. We use our models to explore a number of spectral
regions that are sensitive to metallicity and clean of other spectral features.
The most promising metallicity indicator is an absorption feature between 1935
A and 2020 A, which arises from the blending of numerous Fe III transitions. We
compare our model spectra to observations of two well studied high redshift
star-forming galaxies, MS1512-cB58 (a Lyman break galaxy at z = 2.7276), and
Q1307-BM1163 (a UV-bright galaxy at z = 1.411). The profiles of the
photospheric absorption features observed in these galaxies are well reproduced
by the models. In addition, the metallicities inferred from their equivalent
widths are in good agreement with previous determinations based on interstellar
absorption and nebular emission lines. Our new technique appears to be a
promising alternative, or complement, to established methods which have only a
limited applicability at high redshifts.Comment: 18 pages, 12 figures, accepted for publication in the Astrophysical
Journa
Red supergiants as cosmic abundance probes: The first direct metallicity determination of NGC 4038 in the antennae.
We present a direct determination of the stellar metallicity in the close pair galaxy NGC 4038 (D= 20 Mpc) based on the quantitative analysis of moderate resolution KMOS/VLT spectra of three super star clusters (SSCs). The method adopted in our analysis has been developed and optimised to measure accurate metallicities from atomic lines in the J-band of single red supergiant (RSG) or RSG-dominated star clusters. Hence, our metallicity measurements are not a_ected by the biases and poorly understood systematics inherent to strong line H II methods which are routinely applied to massive data sets of galaxies. We _nd [Z]= +0.07 _ 0.03 and compare our measurements to H II strong line calibrations. Our abundances and literature data suggest the presence of a at metallicity gradient, which can be explained as redistribution of metal-rich gas following the strong interaction
The Evolution of Massive Stars. I. Red Supergiants in the Magellanic Clouds
We investigate the red supergiant (RSG) content of the SMC and LMC using
multi-object spectroscopy on a sample of red stars previously identified by
{\it BVR} CCD photometry. We obtained high accuracy ( km s) radial
velocities for 118 red stars seen towards the SMC and 167 red stars seen
towards the LMC, confirming most of these (89% and 95%, respectively) as red
supergiants (RSGs). Spectral types were also determined for most of these RSGs.
We find that the distribution of spectral types is skewed towards earlier type
at lower metallicities: the average (median) spectral type is K5-7 I in the
SMC, M1 I in the LMC, and M2 I in the Milky Way. We argue that RSGs in the
Magellanic Clouds are 100deg (LMC) and 300deg (SMC) cooler than Galactic RSGs
of the same spectral type. We compare the distribution of RSGs in the H-R
diagram to that of various stellar evolutionary models; we find that none of
the models produce RSGs as cool and luminous as what is actually observed. In
all of our H-R diagrams, however, there is an elegant sequence of decreasing
effective temperatures with increasing luminosities; explaining this will be an
important test of future stellar evolutionary models.Comment: Version with eps figures embedded can be obtained from
ftp://ftp.lowell.edu/pub/massey/rsgs.ps.gz Accepted by the Astronomical
Journa
Red Supergiants as Cosmic Abundance Probes: massive star clusters in M83, and the mass-metallicity relation of nearby galaxies
We present an abundance analysis of seven super-star clusters in the disk of M83. The near-infrared spectra of these clusters are dominated by Red Supergiants, and the spectral similarity in the J-band of such stars at uniform metallicity means that the integrated light from the clusters may be analysed using the same tools as those applied to single stars. Using data from VLT/KMOS we estimate metallicities for each cluster in the sample. We find that the abundance gradient in the inner regions of M83 is flat, with a central metallicity of [Z] = 0.210.11 relative to a Solar value of =0.014, which is in excellent agreement with the results from an analysis of luminous hot stars in the same regions. Compiling this latest study with our other recent work, we construct a mass-metallicity relation for nearby galaxies based entirely on the analysis of RSGs. We find excellent agreement with the other stellar-based technique, that of blue supergiants, as well as with temperature-sensitive (`auroral' or `direct') \hii-region studies. Of all the HII-region strong-line calibrations, those which are empirically calibrated to direct-method studies (N2 and O3N2) provide the most consistent results
On the fine structure of the Cepheid metallicity gradient in the Galactic thin disk
We present homogeneous and accurate iron abundances for 42 Galactic Cepheids
based on high-spectral resolution (R~38,000) high signal-to-noise ratio
(SNR>100) optical spectra collected with UVES at VLT (128 spectra). The above
abundances were complemented with high-quality iron abundances provided either
by our group (86) or available in the literature. We paid attention in deriving
a common metallicity scale and ended up with a sample of 450 Cepheids. We also
estimated for the entire sample accurate individual distances by using
homogeneous near-infrared photometry and the reddening free Period-Wesenheit
relations. The new metallicity gradient is linear over a broad range of
Galactocentric distances (Rg~5-19 kpc) and agrees quite well with similar
estimates available in the literature (-0.060+/-0.002 dex/kpc). We also uncover
evidence which suggests that the residuals of the metallicity gradient are
tightly correlated with candidate Cepheid Groups (CGs). The candidate CGs have
been identified as spatial overdensities of Cepheids located across the thin
disk. They account for a significant fraction of the residual fluctuations, and
in turn for the large intrinsic dispersion of the metallicity gradient. We
performed a detailed comparison with metallicity gradients based on different
tracers: OB stars and open clusters. We found very similar metallicity
gradients for ages younger than 3 Gyrs, while for older ages we found a
shallower slope and an increase in the intrinsic spread. The above findings
rely on homogeneous age, metallicity and distance scales. Finally we found, by
using a large sample of Galactic and Magellanic Cepheids for which are
available accurate iron abundances, that the dependence of the luminosity
amplitude on metallicity is vanishing.Comment: Accepted in A&A, 11 figures, 7 table
- âŠ