No Arabic abstract
Near-infrared imaging surveys of high-mass star-forming regions reveal an amazingly complex interplay between star formation and the environment (Churchwell et al. 2006; Alvarez et al. 2004). By means of near-IR spectroscopy the embedded massive young stars can be characterized and placed in the context of their birth site. However, so far spectroscopic surveys have been hopelessly incomplete, hampering any systematic study of these very young massive stars. New integral field instrumentation available at ESO has opened the possibility to take a huge step forward by obtaining a full spectral inventory of the youngest massive stellar populations in star-forming regions currently accessible. Simultaneously, the analysis of the extended emission allows the characterization of the environmental conditions. The Formation and Early Evolution of Massive Stars (FEMS) collaboration aims at setting up a large observing campaign to obtain a full census of the stellar content, ionized material, outflows and PDRs over a sample of regions that covers a large parameter space. Complementary radio, mm and infrared observations will be used for the characterization of the deeply embedded population. For the first eight regions we have obtained 40 hours of SINFONI observations. In this contribution, we present the first results on three regions that illustrate the potential of this strategy.
We studied the radio properties of very young massive regions of star formation in HII galaxies, with the aim of detecting episodes of recent star formation in an early phase of evolution where the first supernovae start to appear. The observed radio spectral energy distribution (SED) covers a behaviour range; 1) there are galaxies where the SED is characterized by a synchrotron-type slope, 2) galaxies with a thermal slope, and 3) galaxies with possible free-free absorption at long wavelengths. The latter SED represents a signature of massive star clusters that are still well inside the progenitor molecular cloud. Based on the comparison of the star formation rates (SFR) determined from the recombination lines and those determined from the radio emission we find that SFR(Ha) is on average five times higher than SFR(1.4 GHz). These results suggest that the emission of these galaxies is dominated by a recent and massive star formation event in which the first supernovae (SN) just started to explode. We conclude that the systematic lack of synchrotron emission in those systems with the largest equivalent width of Hb can only be explained if those are young starbursts of less than 3.5Myr of age, i.e. before the first type II SNe emerge.
Methanol masers at 6.7 GHz are the brightest of class II methanol masers and have been found exclusively towards massive star forming regions. These masers can thus be used as a unique tool to probe the early phases of massive star formation. We present here a study of the spectral energy distributions of 320 6.7 GHz methanol masers chosen from the MMB catalogue, which fall in the Hi-GAL range ($|l| le 60^{circ}$, $|b| le 1^{circ}$). The spectral energy distributions are constructed from $870 - 70~mu$m using data from the ATLASGAL and Hi-GAL surveys. The emission from cold dust is modelled by a single grey body component fit. We estimate the clump properties such as mass, FIR luminosity and column density using the best fit parameters of the SED fits. Considering the Kauffman criteria for massive star formation, we find that all but a few maser hosts have the potential to harbour at least one high mass star. The physical properties of the methanol maser hosts are also discussed. The evolutionary stages of 6.7 GHz maser sources, explored using the mass luminosity diagram, suggests that they are predominantly associated with high mass stars with the majority being in the accretion phase. However, we observe a small number of sources that could possibly be related to intermediate or low-mass stars.
Our knowledge of the population of young (age <=750 Myr) stars that lie within ~120 pc of the Sun is rapidly accelerating. The vast majority of these nearby, young stars can be placed in kinematically coherent groups (nearby, young moving groups; NYMGs). NYMGs and their member stars afford unmatched opportunities to explore a wide variety of aspects of the early evolution of stars and exoplanet systems, including stellar initial mass functions and age determination methods; the magnetic activities and high-energy radiation environments of young, late-type stars; the dynamics of young binary and hierarchical multiple systems; the late evolutionary stages of circumstellar disks; and, especially, direct-imaging discovery and characterization of massive young exoplanets. In this Astro2020 Science White Paper, we describe how our understanding of these and many other aspects of the early lives of stars and planetary systems is ripe for progress over the next decade via the identification and study of NYMG members with present and next-generation facilities and instruments.
The application of standard accretion theory to observations of X-ray binaries provides valuable insights into neutron star properties, such as their spin period and magnetic field. However, most studies concentrate on relatively old systems, where the neutron star is in its late propeller, accretor, or nearly spin equilibrium phase. Here we use an analytic model from standard accretion theory to illustrate the evolution of high-mass X-ray binaries early in their life. We show that a young neutron star is unlikely to be an accretor because of the long duration of ejector and propeller phases. We apply the model to the recently discovered ~4000 yr old high-mass X-ray binary XMMU J051342.6-672412 and find that the systems neutron star, with a tentative spin period of 4.4 s, cannot be in the accretor phase and has a magnetic field B > (a few)x10^13 G, which is comparable to the magnetic field of many older high-mass X-ray binaries and is much higher than the spin equilibrium inferred value of (a few)x10^11 G. The observed X-ray luminosity could be the result of thermal emission from a young cooling magnetic neutron star or a small amount of accretion that can occur in the propeller phase.
Understanding the different mechanisms of galaxy assembly at various cosmic epochs is a key issue for galaxy evolution and formation models. We present MASSIV (Mass Assembly Survey with SINFONI in VVDS) in this context, an on-going survey with VLT/SINFONI aiming to probe the kinematics and chemical abundances of a unique sample of 84 star-forming galaxies selected in the redshift range z ~ 1-2. This large sample, spanning a wide range of stellar masses, is unique at these high redshifts and statistically representative of the overall galaxy population. In this paper, we give an overview of the MASSIV survey and then focus on the spatially-resolved chemical properties of high-z galaxies and their implication on the process of galaxy assembly.