Masers in star forming regions

Maser emission plays an important role as a tool in star formation studies. It is widely used for deriving kinematics, as well as the physical conditions of different structures, hidden in the dense environment very close to the young stars, for example associated with the onset of jets and outflows. We will summarize the recent observational and theoretical progress on this topic since the last maser symposium: the IAU Symposium 242 in Alice Springs.Comment: Submitted to Proceedings of IAU Symposium No. 287, Cosmic masers - from OH to H

Star-Forming Regions near GRB 990123

We reduced the Hubble Space Telescope Space Telescope Imaging Spectrograph images of the gamma-ray burst GRB 990123 that were obtained on 8-9 February 1999 and find V_0 = 25.36 +/- 0.10, which corresponds to a flux of 0.258 +/- 0.023 micro-Jy for the optical transient 16.644 days after the burst's peak. The probable host galaxy has V_0 = 24.25 +/- 0.07 (= 0.716 +/- 0.046 micro-Jy) and the optical transient is located 0.65 arcseconds (= 5.5 kpc) south of the galaxy's nucleus. We fit and subtracted a scaled point-spread function to the optical transient and found evidence for three bright knots situated within 0.5 arcseconds (= 4.3 kpc) of the optical transient. Each knot has V_0 ~ 28.1 +/- 0.3, a rest-frame V-band luminosity of between approximately 5e8 L_Sun and 8e8 L_Sun, and a star-formation rate of at least 0.1-0.2 Solar masses per year. The knots are centrally concentrated with full-width at half-maximum of approximately 0.17 arcseconds (= 1.5 kpc). Their sizes and luminosities are consistent with their being star-forming regions. The optical transient is located 0.15 arcseconds (= 1.3 kpc) southeast of the centre of one of these knots.Comment: 4 pages, 2 figures, to appear in Astronomy & Astrophysics Letter

Age spreads in star forming regions?

Rotation periods and projected equatorial velocities of pre-main-sequence (PMS) stars in star forming regions can be combined to give projected stellar radii. Assuming random axial orientation, a Monte-Carlo model is used to illustrate that distributions of projected stellar radii are very sensitive to ages and age dispersions between 1 and 10 Myr which, unlike age estimates from conventional Hertzsprung-Russell diagrams, are relatively immune to uncertainties due to extinction, variability, distance etc. Application of the technique to the Orion Nebula cluster reveals radius spreads of a factor of 2--3 (FWHM) at a given effective temperature. Modelling this dispersion as an age spread suggests that PMS stars in the ONC have an age range larger than the mean cluster age, that could be reasonably described by the age distribution deduced from the Hertzsprung-Russell diagram. These radius/age spreads are certainly large enough to invalidate the assumption of coevality when considering the evolution of PMS properties (rotation, disks etc.) from one young cluster to another.Comment: To appear in "The Ages of Stars", E.E. Mamajek, D.R. Soderblom, R.F.G. Wyse (eds.), IAU Symposium 258, CU

Star Formation in Collision Debris: Insights from the modeling of their Spectral Energy Distribution

During galaxy-galaxy interactions, massive gas clouds can be injected into the intergalactic medium which in turn become gravitationally bound, collapse and form stars, star clusters or even dwarf galaxies. The objects resulting from this process are both "pristine", as they are forming their first generation of stars, and chemically evolved because the metallicity inherited from their parent galaxies is high. Such characteristics make them particularly interesting laboratories to study star formation. After having investigated their star-forming properties, we use photospheric, nebular and dust modeling to analyze here their spectral energy distribution (SED) from the far-ultraviolet to the mid-infrared regime for a sample of 7 star-forming regions. Our analysis confirms that the intergalactic star forming regions in Stephan's Quintet, around Arp 105, and NGC 5291, appear devoid of stellar populations older than 10^9 years. We also find an excess of light in the near-infrared regime (from 2 to 4.5 microns) which cannot be attributed to stellar photospheric or nebular contributions. This excess is correlated with the star formation rate intensity suggesting that it is probably due to emission by very small grains fluctuating in temperature as well as the polycyclic aromatic hydrocarbons (PAH) line at 3.3 micron. Comparing the attenuation via the Balmer decrement to the mid-infrared emission allows us to check the reliability of the attenuation estimate. It suggests the presence of embedded star forming regions in NGC 5291 and NGC 7252. Overall the SED of star-forming regions in collision debris (and Tidal Dwarf Galaxies) resemble more that of dusty star-forming regions in galactic disks than to that of typical star-forming dwarf galaxies.Comment: 22 pages, 24 figures, accepted for publication in A

Origin of optically passive spiral galaxies with dusty star-forming regions: Outside-in truncation of star formation?

Recent observations have revealed that red, optically--passive spiral galaxies with little or no optical emission lines, harbour significant amounts of dust-obscured star formation. We propose that these observational results can be explained if the spatial distributions of the cold gas and star-forming regions in these spiral galaxies are significantly more compact than those in blue star-forming spirals. Our numerical simulations show that if the sizes of star-forming regions in spiral galaxies with disk sizes of R_d are ~ 0.3R_d, such galaxies appear to have lower star formation rates as well as higher degrees of dust extinction. This is mainly because star formation in these spirals occurs only in the inner regions where both the gas densities and metallicities are higher, and hence the dust extinction is also significantly higher. We discuss whether star formation occurring preferentially in the inner regions of spirals is closely associated with the stripping of halo and disk gas via some sort of environmental effect. We suggest that the "outside-in truncation of star formation" is the key to a better understanding of apparently optically--passive spirals with dusty star-forming regions.Comment: 5 pages, 4 figures, accepted in MNRAS Letter

The Lifetimes of Phases in High-Mass Star-Forming Regions

High-mass stars form within star clusters from dense, molecular regions, but is the process of cluster formation slow and hydrostatic or quick and dynamic? We link the physical properties of high-mass star-forming regions with their evolutionary stage in a systematic way, using Herschel and Spitzer data. In order to produce a robust estimate of the relative lifetimes of these regions, we compare the fraction of dense, molecular regions above a column density associated with high-mass star formation, N(H2) > 0.4-2.5 x 10^22 cm^-2, in the 'starless (no signature of stars > 10 Msun forming) and star-forming phases in a 2x2 degree region of the Galactic Plane centered at l=30deg. Of regions capable of forming high-mass stars on ~1 pc scales, the starless (or embedded beyond detection) phase occupies about 60-70% of the dense, molecular region lifetime and the star-forming phase occupies about 30-40%. These relative lifetimes are robust over a wide range of thresholds. We outline a method by which relative lifetimes can be anchored to absolute lifetimes from large-scale surveys of methanol masers and UCHII regions. A simplistic application of this method estimates the absolute lifetimes of the starless phase to be 0.2-1.7 Myr (about 0.6-4.1 fiducial cloud free-fall times) and the star-forming phase to be 0.1-0.7 Myr (about 0.4-2.4 free-fall times), but these are highly uncertain. This work uniquely investigates the star-forming nature of high-column density gas pixel-by-pixel and our results demonstrate that the majority of high-column density gas is in a starless or embedded phase.Comment: 10 pages, accepted to Ap