Star formation

Various topics on star formation, centered on the observed properties of young stars and their environment, are reviewed. (a) In our Galaxy, young stellar objects are generally associated with giant molecular clouds. (b) Giant molecular clouds cannot be in free-fall collapse. They are probably stabilized by magnetic fields, which are then likely to dominate the dynamical evolution of the clouds themselves. (c) Star formation occurs mostly in spiral arms. The role of spiral density waves is however not yet clearly understood. (d) The formation of massive stars can perturb the evolution of the progenitor cloud, and possibly trigger the sequential formation of OB subgroups. (e) There is a large number of clouds in the Galaxy associated only with low and intermediate mass young stars. These clouds are not perturbed by the presence of massive stars, and are probably the best source of information on the primary triggering mechanism, active on a galactic scale, and on the initial conditions for star formation.Paper presented at the European Workshop on Planetary Sciences, organised by the Laboratorio di Astrofisica Spaziale di Frascati, and held between April 23–27, 1979, at the Accademia Nazionale del Lincei in Rome, Italy.     

Star formation in taurus

Data with the 2MASS prototype camera were obtained in a 2.3 sq. deg region in Taurus containing Heiles Cloud 2, a region known from IRAS observations to contain a number of very young solar type stars. Data at 1.25 (J), 1.65 (H), and 2.2 (K _s )µm are presented. These data are representative of the type and quality of data expected from the planned near-IR surveys, 2MASS and DENIS. Near-IR surveys will be useful for determining the large scale variation of extinction with clouds, for determining the luminosity function in nearby clouds down to ranges of 0.1–1.0 L, and for finding highly extincted T Tauri stars missed by IRAS because the bulk of their luminosity is emitted shortward of 12µm.     

Star formation region in Vela

A star formation region connected with SNO 41 is investigated. The observations of this region were carried out in the ¹²CO (1-0) line and in the 1.2-mm (with SIMBA) with the 15-m SEST mm telescope (Cerro La Silla, Chile). A blue shifted outflow is revealed from the ¹²CO(1-0) observations, while a bipolar outflow is apparent from the 1.2-mm SIMBA image. In CO it seems that a very faint dust envelope around SNO 41 probably exists, which is expanding with a velocity of ∼10.5 km/s. The distance to SNO 41 is estimated as ∼1500 pc. There are outflows also present in 2MASS images. A spiral jet has a condensation (resembling a HH object) at the end. Another jet has a discontinuity and a bow-shock-like structure on it. In 2MASS images there are also spots resembling HH objects. In this region there is also a rather luminous point source (IRAS 08546-4254), which has IR colors typical for an YSO connected with a water maser. The detection of a strong CS (2-1) line emission toward IRAS 08546-4254, with the same velocity as the CO line, shows the existence of a high density core of molecular gas associated to this source. A methanol maser is also associated with that IRAS source. The existence of CS line emission and a methanol maser (at 6.669 Ghz) is an indication of the presence of a very young massive star. It is not excluded that this IRAS source is the center of outflows mentioned above, because this source coincides with the center of the 1.2-mm SIMBA image and also with the place of origin of the jet with bow-shock-like structure. Published in Astrofizika, Vol. 50, No. 1, pp. 5–15 (February 2007).     

Star formation in shocked layers

We analyze the gravitational stability of a shocked interstellar gas layer and show how such a layer fragments into protostellar condensations whilst it is still confined mainly by ram pressure. As a consequence, the resulting protostars are massive and well separated. Our analysis is completely general and applies both to layers resulting from collisions between molecular cloud clumps, and to shells swept up by expanding nebulae. We present a numerical simulation of the former scenario, which produces a cluster of 35 massive stars resembling an OB subgroup, with most of the stars in binary systems.     

Star formation in NGC5471

We present a study of the star formation in the giant Hii region NGC5471 in the outskirts of M101. We have performed integrated photometry of the mean emission knots. Stellar population analysis with the code chorizos is compatible with the hypothesis that these regions have at least two different populations. The color–magnitude diagram of the region as derived from Hubble Space Telescope/WPFC2 stellar photometry shows that star formation has been going on for the last 70 Myr.     

Star formation in the galaxy

A previous theory of the authors regarding the planetary system is generalized in an attempt to include star formation. It is found that the theory predicts the correct mass and radius for stellar clusters and also the general shape of the galaxy.     

Star formation and AGN

We analyse the data from narrow band H and [O III] CCD imaging of a sample of nearby spiral galaxies with active nuclei. The results show that the large scale star formation depends on the level of nuclear activity, with Seyfert 2 presenting the strongest disk and circum-nuclear star formation, while Seyfert 1 show very little. We present particular results for the galaxies NGC 3982, NGC 4579, NGC 5427 and NGC 7469.     

Star formation in a multi-phase ISM

We present a 3d code for the dynamical evolution of a multi-phase interstellar medium (ISM) coupled to stars via star formation (SF) and feedback processes. The multi-phase ISM consists of clouds (sticky particles) and diffuse gas (SPH): exchange of matter, energy and momentum is achieved by drag (due to ram pressure) and condensation or evaporation processes. The cycle of matter is completed by SF and feedback by SNe and PNe. A SF scheme based on a variable SF efficiency as proposed by Elmegreen and Efremov (1997) is presented. For a Milky Way type galaxy we get a SF rate of ∼1 M_⊙ yr^-1 with an average SF efficiency of ∼5%. This revised version was published online in August 2006 with corrections to the Cover Date.     

Star formation in southern Seyfert galaxies

We have produced radio maps, using the Australia Telescope Compact Array, of the central regions of six southern type 2 Seyfert galaxies (NGC 1365, 4945, 6221, 6810, 7582 and Circinus) with circumnuclear star formation, to estimate the relative contribution of star formation activity compared to activity from the active galactic nucleus (AGN). The radio morphologies range from extended diffuse structures to compact nuclear emission, with no evidence, even in the relatively compact sources, for synchrotron self-absorption. In each case the radio to far-infrared (FIR) ratio has a value consistent with star formation, and in all but one case the radio to [Fe  II ] ratio is also consistent with star formation. We derive supernova rates and conclude that, despite the presence of a Seyfert nucleus in these galaxies, the radio, FIR and [Fe  II ] line emissions are dominated by processes associated with the circumnuclear star formation (i.e. supernova remnants and H  II regions) rather than with the AGN.     

Star formation in nearby isolated galaxies

We use the FUV fluxes measured with the GALEX to study the star formation properties of galaxies collected in the “Local Orphan Galaxies” catalog (LOG). Among 517 LOG galaxies having radial velocities V _LG < 3500 km/s and Galactic latitudes |b| > 15°, 428 objects have been detected in FUV. We briefly discuss some scaling relations between the specific star formation rate (SSFR) and stellar mass, HI-mass, morphology, and surface brightness of galaxies situated in extremely low density regions of the Local Supercluster. Our sample is populated with predominantly late-type, gas-rich objects with the median morphological type of Sdm. Only 5% of LOG galaxies are classified as early types: E, S0, S0/a, however, they systematically differ from normal E and S0 galaxies by lower luminosity and presence of gas and dust. We find that almost all galaxies in our sample have their SSFR below 0.4 [Gyr^?1]. This limit is also true even for a sample of 270 active star-burst Markarian galaxies situated in the same volume. The existence of such a quasi-Eddington limit for galaxies seems to be a key factor which characterizes the transformation of gas into stars at the current epoch.     

Star formation rate in starburst galaxies

The statistical investigation of integrated characteristics of the blue compact galaxies from the Second Byurakan survey has been carried out. The tight correlation between the luminosities in the far-infrared range and radio range (radio-continuum at 21 cm) has been revealed. It implies the common origin of this radiation in star forming regions. The current star formation rates have been derived both from the luminosity in the far-infrared range (SFR_FIR) and from the luminosity in the radio range (SFR_{21 cm}). The Salpeter shape of the initial mass function (IMF) and low-mass cut-off of 0.1 M_⊙ in IMF has been assumed for the SFR_FIR calculation. We derive the SFR_{21 cm} assuming that radio free–free emission at λ=21 cm is thermal emission of gas ionised by massive stars. The SFR_{21 cm} is shown to be about 3.5 times as large as SFR_FIR for our sample of galaxies. These star formation rates are proposed to be considered as lower and upper limits of true galaxy star formation rates.     

Star formation and molecular clouds

The hypothesis advanced by V. A. Ambartsumyan according to which stars are formed from prestellar superdense objects-- protostars-- was an alternative to the hypothesis of the 1950's (and even now, not much changed) according to which stars are formed by accretion with subsequent collapse (in various modifications). Ambartsumyan's basic inferences were based on an analysis of the observational data available at that time. This paper presents both Ambartsumyan's pioneering ideas and some modern hypotheses of star formation. Some results from studies of molecular clouds and star formation regions are also discussed. One of the distinctive features of young stellar objects (YSO) is the outflow of matter from these objects (molecular, in the form of jets, etc.), a phenomenon whose importance for the evolution of stars was noted by Ambartsumyan as long ago as 1937. Radial systems of dark globules are examined, as well as H-H objects associated with star formation regions, cometary nebulae, and close Trapeziumtype systems (consisting of YSO). Translated from Astrofizika, Vol. 52, No. 2, pp. 185–202 (May 2009).     

Star formation and extinct radioactivities

A supernova trigger for star formation, such as was discussed by Cameron and Truran a few years ago, lacks observational confirmation, and the nucleosynthesis arguments for suggesting its relevance to the formation of the solar system no longer seem compelling. Observations of star formation currently point toward nearly isolated individual events taking place in the interiors of dense molecular clouds, in which cores are formed in the clouds and collapse to form stars. If the formation of the solar system proceeded in this manner, then a viable theory of cloud core formation must provide circumstances in which it is not uncommon for the cloud cores to be formed with a significant supply of fresh short-lived radioactivities. A review is made of the evidence for the existence of now extinct radioactivities in primitive solar system material and an examination is made of the implications for the early stages of formation of the Sun and solar system. The characteristics of possible disturbances in dense molecular clouds which can initiate the formation of cloud cores is discussed,and in particular those disturbances which can produce fresh radioactivities are considered. A red giant star of roughly one solar mass on the asymptotic giant branch appears to have been the best candidate to account for the short-lived extinct radioactivities in the early solar system. Star formation itself can generate disturbances through the T Tauri stellar winds and bipolar outflows, and it appears likely that several such disturbances in a chain of star-forming events were needed before the radioactive material was diluted enough to be consistent with the abundances seen in early solar system material.     

Star formation in a multi-phase interstellar medium

This contribution reports on our first efforts to simulate a multiphase interstellar medium on a kiloparsec scale in three dimensions with the stars and gas modeled self-consistently. Starting from inhomogenous initial conditions, our closed box simulations follow the gas as it cools and collapses under its own self-gravity to form stars which eventually return material and energy back through supernovae explosions and winds. This revised version was published online in August 2006 with corrections to the Cover Date.     

Star formation sustained by gas accretion

Numerical simulations predict that metal-poor gas accretion from the cosmic web fuels the formation of disk galaxies. This paper discusses how cosmic gas accretion controls star formation, and summarizes the physical properties expected for the cosmic gas accreted by galaxies. The paper also collects observational evidence for gas accretion sustaining star formation. It reviews evidence inferred from neutral and ionized hydrogen, as well as from stars. A number of properties characterizing large samples of star-forming galaxies can be explained by metal-poor gas accretion, in particular, the relationship among stellar mass, metallicity, and star-formation rate (the so-called fundamental metallicity relationship). They are put forward and analyzed. Theory predicts gas accretion to be particularly important at high redshift, so indications based on distant objects are reviewed, including the global star-formation history of the universe, and the gas around galaxies as inferred from absorption features in the spectra of background sources.     

Star formation by condensation,I

We describe two possibilities for the formation of a star by transition of molecular hydrogen from gaseous to the liquid phase in the gravitational field of a spherical mass.Model A.: Contraction through condensation of a polytropic respectively isothermal gas sphere.Model B: Condensation in the isothermal atmosphere which surrounds a core.Model B is sooner possible, because model A introduces certain restrictions of mass and radius of the protostar.     

Star formation histories across hubble types

Using population fitting and galactic chemical evolution (GCE) models we unravel the information present in spectral line-strengths, for a sample of 32 galaxies including ellipticals, S0s and spiral bulges. This revised version was published online in August 2006 with corrections to the Cover Date.