15 research outputs found
The Nuclear Stellar Cluster in the Seyfert~1 Galaxy NGC 3227: High Angular Resolution NIR Imaging and Spectroscopy
NIR high angular resolution speckle imaging and imaging spectroscopy of the
nuclear region (10'' ~ 840pc) of the Seyfert1 galaxy NGC3227 are presented. A
nuclear stellar cluster is slightly resolved in the J and H band with
increasing contribution to the NIR continuum from the K to the J band. The
stellar absorption lines are extended compared to the neighboring continuum
suggesting a cluster size of ~ 70pc FWHM. Analysis of those lines suggests that
the stars are contributing about 65% (40%) of the total continuum emission in
the H (K) band in a 3.6'' aperture. Population synthesis in conjunction with
NIR spectral synthesis indicates an age of 25 to 50 Myr when red supergiants
contribute most to the NIR light. This is supported by published optical data
on the MgIb line and the CaII triplet. Although a higher age of ~ 0.5 Gyr where
AGB stars dominate the NIR light can not be excluded, the observed parameters
are at the limit of those expected for a cluster dominated by AGB stars.
However, in either case the resolved stellar cluster contributes only about ~
15 % of the total dynamical mass in the inner 300pc implying another much older
stellar population. Pure constant star formation over the last 10 Gyr can be
excluded. Therefore, at least two star formation/starburst events took place in
the nucleus of NGC3227. Since such sequences in the nuclear star formation
history are also observed in the nuclei of other galaxies a link between the
activity of the star formation and the AGN itself seems likely.Comment: accepted for publication in the Astrophysical Journal, 46 pages, 15
figure
Calibration of Nebular Emission-Line Diagnostics: I. Stellar Effective Temperatures
We present a detailed comparison of optical H II region spectra to
photoionization models based on modern stellar atmosphere models. We examine
both spatially resolved and integrated emission-line spectra of the HII regions
DEM L323, DEM L243, DEM L199, and DEM L301 in the Large Magellanic Cloud. The
published spectral classifications of the dominant stars range from O7 to WN3,
and morphologies range from Stromgren sphere to shell structure. Two of the
objects include SNR contamination. The overall agreement with the predictions
is generally within 0.2 dex for major diagnostic line ratios. An apparent
pattern in the remaining discrepancies is that the predicted T_e is ~1000 K
hotter than observed. (Abridged)
Our analysis of the complex DEM L199 allows a nebular emission-line test of
unprecedented detail for WR atmospheres. Surprisingly, we find no nebular He II
4686 emission, despite the fact that both of the dominant WN3 stars should be
hot enough to fully ionize He I in their atmospheres.
We confirm that the \eta-prime emission-line parameter is not as useful as
hoped for determining the ionizing stellar effective temperature, T*. Both
empirically and theoretically, we find that it is insensitive for T* >40 kK,
and that it also varies spatially. The shock-contaminated objects show that
\eta-prime will also yield a spuriously high T* in the presence of shocks. It
is furthermore sensitive to shell morphology. We suggest [Ne III]/Hb as an
additional probe of T*. Although it is abundance-dependent, [Ne III]/Hb has
higher sensitivity to T*, is independent of morphology, and is insensitive to
shocks in our objects. We attempt a first empirical calibration of these
nebular diagnostics of T*.Comment: Accepted to ApJS. 37 pages, 14 figures, including 12 jpeg files. Uses
emulateapj Latex style file. Single PS file preprint available at
http://www.stsci.edu/~oey, along with unabridged abstrac
The R136 star cluster dissected with Hubble Space Telescope/STIS. I. Far-ultraviolet spectroscopic census and the origin of HeII 1640 in young star clusters
We introduce a HST/STIS stellar census of R136a, the central ionizing star
cluster of 30 Doradus. We present low resolution far-ultraviolet STIS/MAMA
spectroscopy of R136 using 17 contiguous 52x0.2 arcsec slits which together
provide complete coverage of the central 0.85 parsec (3.4 arcsec). We provide
spectral types of 90% of the 57 sources brighter than m_F555W = 16.0 mag within
a radius of 0.5 parsec of R136a1, plus 8 additional nearby sources including
R136b (O4\,If/WN8). We measure wind velocities for 52 early-type stars from CIV
1548-51, including 16 O2-3 stars. For the first time we spectroscopically
classify all Weigelt & Baier members of R136a, which comprise three WN5 stars
(a1-a3), two O supergiants (a5-a6) and three early O dwarfs (a4, a7, a8). A
complete Hertzsprung-Russell diagram for the most massive O stars in R136 is
provided, from which we obtain a cluster age of 1.5+0.3_-0.7 Myr. In addition,
we discuss the integrated ultraviolet spectrum of R136, and highlight the
central role played by the most luminous stars in producing the prominent HeII
1640 emission line. This emission is totally dominated by very massive stars
with initial masses above ~100 Msun. The presence of strong HeII 1640 emission
in the integrated light of very young star clusters (e.g A1 in NGC 3125)
favours an initial mass function extending well beyond a conventional upper
limit of 100 Msun. We include montages of ultraviolet spectroscopy for LMC O
stars in the Appendix. Future studies in this series will focus on optical
STIS/CCD medium resolution observations.Comment: 20 pages plus four Appendices providing LMC UV O spectral templates,
UV spectral atlas in R136, wind velocities of LMC O stars and photometry of
additional R136 source
The R136 star cluster dissected with Hubble Space Telescope/STIS. I. Far-ultraviolet spectroscopic census and the origin of He II lambda 1640 in young star clusters
We introduce a Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph (STIS) stellar census of R136a, the central ionizing star cluster of 30 Doradus. We present low resolution far-ultraviolet STIS spectroscopy of R136 using 17 contiguous 52 arcsec Ă 0.2 arcsec slits which together provide complete coverage of the central 0.85 parsec (3.4 arcsec). We provide spectral types of 90 per cent of the 57 sources brighter than mF555W = 16.0 mag within a radius of 0.5 parsec of R136a1, plus 8 additional nearby sources including R136b (O4 If/WN8). We measure wind velocities for 52 early-type stars from CâIVΝΝ1548â51, including 16 O2â3 stars. For the first time, we spectroscopically classify all Weigelt and Baier members of R136a, which comprise three WN5 stars (a1âa3), two O supergiants (a5âa6) and three early O dwarfs (a4, a7, a8). A complete HertzsprungâRussell diagram for the most massive O stars in R136 is provided, from which we obtain a cluster age of 1.5
+0.3â0.7
â0.7+0.3
Myr. In addition, we discuss the integrated ultraviolet spectrum of R136, and highlight the central role played by the most luminous stars in producing the prominent HeâII Îť1640 emission line. This emission is totally dominated by very massive stars with initial masses above âź100âMâ. The presence of strong HeâII Îť1640 emission in the integrated light of very young star clusters (e.g. A1 in NGC 3125) favours an initial mass function extending well beyond a conventional upper limit of 100âMâ. We include montages of ultraviolet spectroscopy for Large Magellanic Cloud O stars in the appendix. Future studies in this series will focus on optical STIS medium resolution observations
The Global Content, Distribution, and Kinematics of Interstellar O VI in the Large Magellanic Cloud
We present FUSE observations of interstellar O VI absorption towards 12
early-type stars in the Large Magellanic Cloud (LMC). O VI 1031.926 Ang
absorption at LMC velocities is seen towards all 12 stars. The observed columns
are in the range log N(O VI)=13.9 to 14.6, with a mean of 14.37. The
observations probe several sight lines projected onto known superbubbles, but
these show relatively little (if any) enhancement in O VI column density
compared to sight lines towards relatively quiescent regions of the LMC. The
observed LMC O VI absorption is broad, with Gaussian dispersions of 30 to 50
km/sec, implying temperatures T<(2-5)x10^6 K. The O VI absorption is typically
displaced -30 km/sec from the corresponding low-ionization absorption
associated with the bulk of the LMC gas. The properties of the LMC O VI
absorption are very similar to those of the Milky Way halo. The average column
density of O VI and the dispersion of the individual measurements about the
mean are identical to those measured for the halo of the Milky Way, even though
the metallicity of the LMC is a factor of ~2.5 lower than the Milky Way. Much
of the LMC O VI may arise in a vertically-extended distribution similar to the
Galactic halo. If the observed O VI absorption is tracing a radiatively cooling
galactic fountain flow, the mass flow rate from one side of the LMC disk is of
the order 1 Msun/yr, with a mass flux per unit area of the disk ~0.02
Msun/yr/kpc^2. (abridged)Comment: Accepted for publiction in the ApJ. 39 pages, including 9 figures and
6 tables. Version with full resolution figures available at
http://fuse.pha.jhu.edu/~howk/Papers
The VLT-FLAMES Tarantula Survey: XXX. Red stragglers in the clusters Hodge 301 and SL 639
Aims: We estimate physical parameters for the late-type massive stars observed as part of the VLT-FLAMES Tarantula Survey (VFTS) in the 30 Doradus region of the Large Magellanic Cloud (LMC).
Methods: The observational sample comprises 20 candidate red supergiants (RSGs) which are the reddest ((Bâ
ââ
V) > 1 mag) and brightest (V < 16 mag) objects in the VFTS. We use optical and near-infrared (near-IR) photometry to estimate their temperatures and luminosities, and introduce the luminosityâage diagram to estimate their ages.
Results: We derive physical parameters for our targets, including temperatures from a new calibration of (Jâ
ââ
Ks)0 colour for luminous cool stars in the LMC, luminosities from their J-band magnitudes (thence radii), and ages from comparisons with current evolutionary models. We show that interstellar extinction is a significant factor for our targets, highlighting the need to take it into account in the analysis of the physical parameters of RSGs. We find that some of the candidate RSGs could be massive AGB stars. The apparent ages of the RSGs in the Hodge 301 and SL 639 clusters show a significant spread (12â24 Myr). We also apply our approach to the RSG population of the relatively nearby NGC 2100 cluster, finding a similarly large spread.
Conclusions We argue that the effects of mass transfer in binaries may lead to more massive and luminous RSGs (which we call âred stragglersâ) than expected from single-star evolution, and that the true cluster ages correspond to the upper limit of the estimated RSG ages. In this way, the RSGs can serve as a new and potentially reliable age tracer in young star clusters. The corresponding analysis yields ages of 24-3+5 Myr for Hodge 301, 22-5+6 Myr for SL 639, and 23-2+4 Myr for NGC 2100
The VLT-FLAMES Tarantula Survey. XXI. Stellar spin rates of O-type spectroscopic binaries
Context. The initial distribution of spin rates of massive stars is a fingerprint of their elusive formation process. It also sets a key initial condition for stellar evolution and is thus an important ingredient in stellar population synthesis. So far, most studies have focused on single stars. Most O stars are, however, found in multiple systems.
Aims: By establishing the spin-rate distribution of a sizeable sample of O-type spectroscopic binaries and by comparing the distributions of binary subpopulations with one another and with that of presumed-single stars in the same region, we aim to constrain the initial spin distribution of O stars in binaries, and to identify signatures of the physical mechanisms that affect the evolution of the spin rates of massive stars.
Methods: We use ground-based optical spectroscopy obtained in the framework of the VLT-FLAMES Tarantula Survey (VFTS) to establish the projected equatorial rotational velocities (νesini) for components of 114 spectroscopic binaries in 30 Doradus. The νesini values are derived from the full width at half maximum (FWHM) of a set of spectral lines, using a FWHM vs. νesini calibration that we derive based on previous line analysis methods applied to single O-type stars in the VFTS sample.
Results: The overall νesini distribution of the primary stars resembles that of single O-type stars in the VFTS, featuring a low-velocity peak (at νesini< 200 kms-1) and a shoulder at intermediate velocities (200 < νesini< 300 kms-1). The distributions of binaries and single stars, however, differ in two ways. First, the main peak at νesini ~ 100kms-1 is broader and slightly shifted towards higher spin rates in the binary distribution than that of the presumed-single stars. This shift is mostly due to short-period binaries (Porb~< 10 d). Second, the νesini distribution of primaries lacks a significant population of stars spinning faster than 300 kms-1, while such a population is clearly present in the single-star sample. The νesini distribution of binaries with amplitudes of radial velocity variation in the range of 20 to 200 kms-1 (mostly binaries with Porb ~ 10-1000 d and/or with q< 0.5) is similar to that of single O stars below νesini~< 170kms-1.
Conclusions: Our results are compatible with the assumption that binary components formed with the same spin distribution as single stars, and that this distribution contains few or no fast-spinning stars. The higher average spin rate of stars in short-period binaries may either be explained by spin-up through tides in such tight binary systems, or by spin-down of a fraction of the presumed-single stars and long-period binaries through magnetic braking (or by a combination of both mechanisms). Most primaries and secondaries of SB2 systems with Porb~ 300kms-1 in the single-star sample are actually spun-up post-binary interaction products. Finally, the overall similarities (low-velocity peak and intermediate-velocity shoulder) of the spin distribution of binary and single stars argue for a massive star formation process in which the initial spin is set independently of whether stars are formed as single stars or as components of a binary system.16 pages, 16 figures, paper accepted in Astronomy & Astrophysicsstatus: publishe