An HST/NICMOS view of the prototypical giant HII region NGC604 in M33

We present the first high-spatial resolution near-infrared (NIR) imaging of NGC 604, obtained with the NICMOS camera aboard the Hubble Space Telescope (HST). These NICMOS broadband images reveal new NIR point sources, clusters, and diffuse structures. We found an excellent spatial correlation between the 8.4 GHz radio continuum and the 2.2mu-m nebular emission. Moreover, massive young stellar object candidates appear aligned with these radio peaks, reinforcing the idea that those areas are star-forming regions. Three different scaled OB associations are recognized in the NICMOS images. The brightest NIR sources in our images have properties that suggest that they are red supergiant stars, of which one of them was previously known. This preliminary analysis of the NICMOS images shows the complexity of the stellar content of the NGC 604 nebula.Comment: Paper presented in the Workshop "Young massive star clusters: initial conditions and environments" (Granada, Spain - Sept 2007). Astrophysics & Space Science in press, 7 pages, 4 figure

The young stellar population of NGC 4214 as observed with HST. I. Data and methods

We present the data and methods that we have used to perform a detailed UV-optical study of the nearby dwarf starburst galaxy NGC 4214 using multifilter HST/WFPC2+STIS photometry. We explain the process followed to obtain high-quality photometry and astrometry of the stellar and cluster populations of this galaxy. We describe the procedure used to transform magnitudes and colors into physical parameters using spectral energy distributions. The data show the existence of both young and old stellar populations that can be resolved at the distance of NGC 4214 (2.94 Mpc) and we perform a general description of those populations.Comment: 33 pages, 9 figures, and 8 table

The spatial distribution of O-B5 stars in the solar neighborhood as measured by Hipparcos

We have developed a method to calculate the fundamental parameters of the vertical structure of the Galaxy in the solar neighborhood from trigonometric parallaxes alone. The method takes into account Lutz-Kelker-type biases in a self-consistent way and has been applied to a sample of O-B5 stars obtained from the Hipparcos catalog. We find that the Sun is located 24.2 +/- 1.7 (random) +/- 0.4 (systematic) pc above the galactic plane and that the disk O-B5 stellar population is distributed with a scale height of 34.2 +/- 0.8 (random) +/- 2.5 (systematic) pc and an integrated surface density of (1.62 +/- 0.04 (random) +/- 0.14 (systematic)) 10^{-3} stars pc^{-2}. A halo component is also detected in the distribution and constitutes at least ~5% of the total O-B5 population. The O-B5 stellar population within ~100 pc of the Sun has an anomalous spatial distribution, with a less-than-average number density. This local disturbance is probably associated with the expansion of Gould's belt.Comment: 14 pages, 3 figures, to appear in the May 2001 issue of the Astronomical Journa

The VLT-FLAMES Tarantula Survey. VII. A low velocity dispersion for the young massive cluster R136

Detailed studies of resolved young massive star clusters are necessary to determine their dynamical state and evaluate the importance of gas expulsion and early cluster evolution. In an effort to gain insight into the dynamical state of the young massive cluster R136 and obtain the first measurement of its velocity dispersion, we analyse multi-epoch spectroscopic data of the inner regions of 30 Doradus in the Large Magellanic Cloud (LMC) obtained as part of the VLT-FLAMES Tarantula Survey. Following a quantitative assessment of the variability, we use the radial velocities of non-variable sources to place an upper limit of 6 km/s on the line-of-sight velocity dispersion of stars within a projected distance of 5 pc from the centre of the cluster. After accounting for the contributions of undetected binaries and measurement errors through Monte Carlo simulations, we conclude that the true velocity dispersion is likely between 4 and 5 km/s given a range of standard assumptions about the binary distribution. This result is consistent with what is expected if the cluster is in virial equilibrium, suggesting that gas expulsion has not altered its dynamics. We find that the velocity dispersion would be ~25 km/s if binaries were not identified and rejected, confirming the importance of the multi-epoch strategy and the risk of interpreting velocity dispersion measurements of unresolved extragalactic young massive clusters.Comment: 18 pages, 7 figures, accepted by A&

The VLT-FLAMES Tarantula Survey XXII. Multiplicity properties of the B-type stars

We investigate the multiplicity properties of 408 B-type stars observed in the 30 Doradus region of the Large Magellanic Cloud with multi-epoch spectroscopy from the VLT-FLAMES Tarantula Survey (VFTS). We use a cross-correlation method to estimate relative radial velocities from the helium and metal absorption lines for each of our targets. Objects with significant radial-velocity variations (and with an amplitude larger than 16 km/s) are classified as spectroscopic binaries. We find an observed spectroscopic binary fraction (defined by periods of 0.1) for the B-type stars, f_B(obs) = 0.25 +/- 0.02, which appears constant across the field of view, except for the two older clusters (Hodge 301 and SL 639). These two clusters have significantly lower fractions of 0.08 +/- 0.08 and 0.10 +/- 0.09, respectively. Using synthetic populations and a model of our observed epochs and their potential biases, we constrain the intrinsic multiplicity properties of the dwarf and giant (i.e. relatively unevolved) B-type stars in 30 Dor. We obtain a present-day binary fraction f_B(true) = 0.58 +/- 0.11, with a flat period distribution. Within the uncertainties, the multiplicity properties of the B-type stars agree with those for the O stars in 30 Dor from the VFTS.Comment: Accepted by A&

The VLT-FLAMES Tarantula Survey XVI. The optical+NIR extinction laws in 30 Doradus and the photometric determination of the effective temperatures of OB stars

Context: The commonly used extinction laws of Cardelli et al. (1989) have limitations that, among other issues, hamper the determination of the effective temperatures of O and early B stars from optical+NIR photometry. Aims: We aim to develop a new family of extinction laws for 30 Doradus, check their general applicability within that region and elsewhere, and apply them to test the feasibility of using optical+NIR photometry to determine the effective temperature of OB stars. Methods: We use spectroscopy and NIR photometry from the VLT-FLAMES Tarantula Survey and optical photometry from HST/WFC3 of 30 Doradus and we analyze them with the software code CHORIZOS using different assumptions such as the family of extinction laws. Results: We derive a new family of optical+NIR extinction laws for 30 Doradus and confirm its applicability to extinguished Galactic O-type systems. We conclude that by using the new extinction laws it is possible to measure the effective temperatures of OB stars with moderate uncertainties and only a small bias, at least up to E(4405-5495) ~ 1.5 mag.Comment: Accepted for publication in A&A. Revised version corrects language and fixes typos (one of them caught by David Nicholls). Figure 4 has poor quality due to the size restrictions imposed by arXi

Tip of the Red Giant Branch Distances to NGC 4214, UGC 685, and UGC 5456

We have used WFPC2 VRI observations to calculate the distances to three nearby galaxies, NGC 4214, UGC 685, and UGC 5456 using the tip of the red giant branch method. Our values for NGC 4214 (2.94 +/- 0.18 Mpc) and UGC 685 (4.79 +/- 0.30 Mpc) are the most precise measurementes of the distances to these objects ever made. For UGC 5456 the data do not allow us to reach a decisive conclusion since there are two possible solutions, one leading towards a short distance around 3.8 Mpc and another one towards a long distance of 5.6 Mpc or more.Comment: To appear in the March 2002 issue of the Astronomical Journal. 19 pages, including 4 tables and 8 figures. Due to the size limitations of the astro-ph service, a reduced resolution version of figures 1-3 is provided here. For a full resolution version, please go to http://www.stsci.edu/~jmai

Luminosity function of faint Galactic sources in the Chandra bulge field

We study the statistical properties of faint X-ray sources detected in the Chandra Bulge Field. The unprecedented sensitivity of the Chandra observations allows us to probe the population of faint Galactic X-ray sources down to luminosities L(2-10 keV)~1e30 erg/sec at the Galactic Center distance. We show that the luminosity function of these CBF sources agrees well with the luminosity function of sources in the Solar vicinity (Sazonov et al. 2006). The cumulative luminosity density of sources detected in the CBF in the luminosity range 1e30-1e32 erg/sec per unit stellar mass is L(2-10 keV)/M*=(1.7+/-0.3)e27 erg/sec/Msun. Taking into account sources in the luminosity range 1e32-1e34 erg/sec from Sazonov et al. (2006), the cumulative luminosity density in the broad luminosity range 1e30-1e34 erg/sec becomes L(2-10 keV)/M*=(2.4+/-0.4)e27 erg/sec/Msun. The majority of sources with the faintest luminosities should be active binary stars with hot coronae based on the available luminosity function of X-ray sources in the Solar environment.Comment: 5 pages, 4 figures, Accepted for publication in MNRA

The VLT-FLAMES Tarantula Survey III: A very massive star in apparent isolation from the massive cluster R136

VFTS 682 is located in an active star-forming region, at a projected distance of 29 pc from the young massive cluster R136 in the Tarantula Nebula of the Large Magellanic Cloud. It was previously reported as a candidate young stellar object, and more recently spectroscopically revealed as a hydrogen-rich Wolf-Rayet (WN5h) star. Our aim is to obtain the stellar properties, such as its intrinsic luminosity, and to investigate the origin of VFTS 682. To this purpose, we model optical spectra from the VLT-FLAMES Tarantula Survey with the non-LTE stellar atmosphere code CMFGEN, as well as the spectral energy distribution from complementary optical and infrared photometry. We find the extinction properties to be highly peculiar (RV ~4.7), and obtain a surprisingly high luminosity log(L/Lsun) = 6.5 \pm 0.2, corresponding to a present-day mass of ~150Msun. The high effective temperature of 52.2 \pm 2.5kK might be explained by chemically homogeneous evolution - suggested to be the key process in the path towards long gamma-ray bursts. Lightcurves of the object show variability at the 10% level on a timescale of years. Such changes are unprecedented for classical Wolf-Rayet stars, and are more reminiscent of Luminous Blue Variables. Finally, we discuss two possibilities for the origin of VFTS 682: (i) the star either formed in situ, which would have profound implications for the formation mechanism of massive stars, or (ii) VFTS 682 is a slow runaway star that originated from the dense cluster R136, which would make it the most massive runaway known to date.Comment: 5 pages, 5 figures, accepted by A&A Letter