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A gas-grain time dependent chemical code, UCL_CHEM, has been used to investigate the possibility of using chemical tracers to differentiate between the possible formation mechanisms of brown dwarfs. We model the formation of a pre-brown dwarf core through turbulent fragmentation by following the depth-dependent chemistry in a molecular cloud through the step change in density associated with an isothermal shock and the subsequent freefall collapse once a bound core is produced. Trends in the fractional abundance of molecules commonly observed in star forming cores are then explored to find a diagnostic for identifying brown dwarf mass cores formed through turbulence. We find that the cores produced by our models would be bright in CO and NH3 but not in HCO+. This differentiates them from models using purely freefall collapse as such models produce cores that would have detectable transitions from all three molecules.
The formation mechanism of brown dwarfs (BDs) is one of the long-standing problems in star formation because the typical Jeans mass in molecular clouds is too large to form these substellar objects. To answer this question, it is crucial to study a BD at the embedded phase (proto-brown dwarf). IRAS16253 is classified as a Very Low Luminosity Object (VeLLO, Lint < 0.1L⊙), which is considered as a proto-brown dwarf candidate. We use the IRAM 30m, APEX telescopes and the SMA to probe the molecular jet/outflow driven by IRAS 16253 in CO (2–1), (6–5), and (7–6) and study its dynamical features and physical properties. We detect a wiggling pattern in the position-velocity diagrams of the jets. Assuming that this pattern is due to the orbital motion of a binary system, we obtain the current mass of the binary is ~0.026 M⊙. Together with the low parent core mass, IRAS16253 will likely form one or two proto-BD in the future. This is the first time that the current mass of a proto-BD binary system is identified through the dynamics of the jets. Since IRAS16253 is located in an isolated environment, we suggest that BDs can form through fragmentation and collapse like low mass stars.
Atomic hydrogen traces the raw material from which molecular clouds and stars form. With 565 galaxies from the ALFALFA Hα survey, a statistically complete subset of the ALFALFA survey, we examine the processes that affect galaxies' abilities to access and consume their Hi gas. On galaxy-wide scales, Hi gas fractions correlate only weakly with instantaneous specific star formation rates (sSFRs) but tightly with galaxy color. We show that a connection between dust and Hi content, arising from the fundamental mass-metallicity-Hi relation, leads to this tight color correlation. We find that disk galaxies follow a relation between stellar surface density and Hi depletion time, consistent with a scenario in which higher mid-plane pressure leads to more efficient molecular cloud formation from Hi. In contrast, spheroids show no such trend. Starbursts, identified by Hα equivalent width, do not show enhanced Hi gas fractions relative to similar mass non-starburst galaxies. The starbursts' shorter Hi depletion times indicate more efficient consumption of Hi, and galaxy interactions drive this enhanced star formation efficiency in several starbursts. Interestingly, the most disturbed starbursts show greater enhancements in Hi gas fraction, which may indicate an excess of Hi at early merger stages. At low galaxy stellar masses, the triggering mechanism for starbursts is less clear; the high scatter in efficiency and sSFR among low-mass galaxies may result from periodic bursts. We find no evidence for depleted Hi reservoirs in starbursts, which suggests that galaxies may maintain sufficient Hi to fuel multiple starburst episodes.
We use three-dimensional magnetohydrodynamic (MHD) simulations to investigate the quasi-equilibrium states of galactic disks regulated by star formation feedback. We incorporate effects from massive-star feedback via time-varying heating rates and supernova (SN) explosions. We find that the disks in our simulations rapidly approach a quasi-steady state that satisfies vertical dynamical equilibrium. The star formation rate (SFR) surface density self-adjusts to provide the total momentum flux (pressure) in the vertical direction that matches the weight of the gas. We quantify feedback efficiency by measuring feedback yields, ηc≡ Pc/ΣSFR (in suitable units), for each pressure component. The turbulent and thermal feedback yields are the same for HD and MHD simulations, ηth ~ 1 and ηturb ~ 4, consistent with the theoretical expectations. In MHD simulations, turbulent magnetic fields are rapidly generated by turbulence, and saturate at a level corresponding to ηmag,t ~ 1. The presence of magnetic fields enhances the total feedback yield and therefore reduces the SFR, since the same vertical support can be supplied at a smaller SFR. We suggest further numerical calibrations and observational tests in terms of the feedback yields.
In order to better understand the chemical conditions and evolutionary properties of massive star-forming regions, and to explore the physical and chemical behavior of simple hydrocarbon molecules, we have used telescopes such as CSO, JCMT, CARMA and SMA, to map the multi-transitions of C2H and HC3N. The column densities and abundances are compared with chemical models to gain some diagnostic of the environment of the regions.
We apply the Jeans Axisymmetric Multi-Gaussian Expansion method to the stellar kinematic maps of 40 Sa–Sd EDGE-CALIFA galaxies and derive their circular velocity curves (CVCs). The CVCs are classified using the Dynamical Classification method developed in Kalinova et al. (2015). We also calculate the observational baryon efficiency, OBE, where M*/Mb=M*/(M*+MHI+MH2) of the galaxies using their stellar mass, total neutral hydrogen mass and total molecular gas from CO luminosities. Slow-rising, Flat and Round-peaked CVC types correspond to specific OBEs, stellar and dark matter (DM) halo mass values, while the Sharp-peaked CVCs span in the whole DM halo mass range of 1011-1014M⊙.
A robust feature of turbulent fragmentation theories is a universal Salpeter like slope (2.2 -2.4), for the mass spectrum of the fragments, at the high mass end. This is so due to the scale-free nature of turbulence and gravity. There are reports of top heavy / flatter Initial Mass Functions (IMF), inferred for many regions where we expect star formation to take place in gas clouds with comparatively higher gas density. Also, a higher Star Formation Efficiency (SFE) for regions of higher gas density has been proposed, to understand the formation of bound stellar systems in which dark matter is not a significant factor affecting the internal dynamics. In turbulent fragmentation models for star formation, we do not expect the mass of the stellar cluster to influence the maximum stellar mass directly and thereby imply a relation between the maximum stellar mass and the cluster mass. However, such a relation may be expected from statistical considerations. In this context, we explore the density dependence of the IMF, that would arise due to denser clouds producing more massive clusters due to the density dependence of the SFE.
We study star formation occurring in nuclear rings of barred-spiral galaxies by using hydrodynamic simulations with the prescriptions of star formation and feedback included. In models without spiral arms, the star formation rate (SFR) in a ring exhibits a strong primary burst at early time and declines to small values at late time. The early burst is caused by a rapid gas infall due to the bar growth, consuming most of the gas inside the bar regions. On the other hand, models with spiral arms show multiple starburst activities at late time caused by arm-induced gas inflows, provided that the arm pattern speed is slower than that of the bar. The SFR in models with spirals is larger by a factor of ~ 1.4–4.0 than that in the bar-only models, with larger values corresponding to stronger and slower arms. In all models, young star clusters in nuclear ring show an azimuthal age gradient only when the SFR is small, such that younger clusters tend to locate closer to the contact points between the ring and dust lanes.
During the last two decade, observations have shown the potential of molecular tracers to get insights into the physical processes taking place in the central regions of active galaxies. However, observations were severely limited by both sensitivity and resolution. This resulted also in a limited sample of bright enough galaxies where molecular species other than carbon monoxide could be observed. Current instruments like ALMA and the upcoming NOEMA are already changing our view of the extragalactic ISM molecular observations. In fact, it is now possible to study the physical properties of individual spatially resolved star forming GMCs in external galaxies, as well as resolving the physical structure of the ISM in the surroundings of AGNs at scales of a few parsecs. Here I quickly review some of the most recent observational studies in the nuclear regions of galaxies which are setting new standards in the ways we can study the extragalactic ISM properties.
Star-formation regions in nearby galaxies provide an excellent laboratory to study star formation processes, evolution of massive stars and the properties of the surrounding interstellar medium. A wealth of information can be obtained from their spectral analysis of the emission lines and the stellar continuum. Considering these, we proposed a long-term project “Spectroscopic Observations of the Star Formation Regions in Nearby Galaxies”. The primary goal of this project is to observe spectroscopy of star formation regions in 20 nearby galaxies, with the NAOC 2.16 m telescope and the Hectospec/MMT multifiber spectrograph. With the spectra of a large sample of star formation regions, combining multi-wavelength data from UV to IR, we can investigate, understand and quantify the dust extinction, star formation rate, metal abundance, and the two-dimensional distributions of stellar population properties of nearby galaxies. It will be important for a better understanding of galaxy formation. Here we report on the observations, data reduction, and analysis of the spectra of ~ 400 star formation regions in M33, via multifiber spectroscopy with Hectospec at the MMT.
One of the main scientific goals of the Herschel Gould Belt survey is to elucidate the physical mechanisms responsible for the formation and evolution of prestellar cores in molecular clouds. In the ~11 deg2 field of Aquila imaged with Herschel/PACS-SPIRE at 70–500 μm, we have identified a complete sample of 651 starless cores, 446 of them are gravitationally-bound candidate prestellar cores. Our Herschel observations also provide an unprecedented census of filaments in the Aquila cloud and suggest an intimate connection between these filaments and the formation process of prestellar cores. Indeed, a strong correlation is found between their spatial distributions. These Herschel findings support a filamentary paradigm for the early stages of star formation, where the cores result from the gravitational fragmentation of the densest filaments.
We have made near-infrared (JHKs) imaging polarimetry toward 24 bright-rimmed clouds in the southern hemisphere in order to reveal their magnetic field structures. The obtained polarization vector maps show that the magnetic field directions inside the bright rim are different from its ambient magnetic field direction, implying that magnetic field structures just inside the ionized front of the clouds are due to the gas compression by UV radiation from nearby massive star. Our investigation into the relation between the magnetic field configuration and the shape of the cloud suggests that the magnetic field configuration affects the evolution of the cloud shape.
We present results of Herschel PACS imaging spectroscopy data toward ten massive young stellar objects taken as part of the WISH project. Our sample consists of four high mass protostellar objects (HMPOs), two hot molecular cores (HMCs), and four ultracompact HII regions (UCHIIs), and the spectra cover a broad range of wavelengths (55 to 210 μm) imaged over an ~50” field with 5×5 spaxels. By fitting the continua utilizing a modified black-body formula we estimate mass-weighted dust temperature and column density distributions of warm dust and find that UCHII regions are hottest and HMCs are most deeply embedded. We also estimate rotational temperature and column density distributions of warm CO gas using the rotational diagram analysis, which are comparable over targets in contrast to continuum results. By comparing high J CO line fluxes to the RATRAN estimates of centrally heated envelope models, we find that majority of warm CO originates from bipolar outflow shocks.
As part of the Herschel Gould Belt Survey (HGBS), the Ophiuchus molecular cloud was imaged in the submillimeter range. Here, we summarize and briefly discuss the main results.
FeLoBAL quasars may be an early evolutionary stage of merger-triggered quasar activity. We test this hypothesis using HST imaging of four FeLoBAL quasars. The host galaxy colors are consistent with early-type quiescent galaxies or heavily-obscured starbursts.
We characterise steady, one-dimensional fast and slow magnetohydrodynamic (MHD) shocks using a two-fluid model. Fast MHD shocks are magnetically driven, forcing ions to stream through the neutral gas ahead of the shock front. This magnetic precursor heats the gas sufficiently to create a large, warm transition zone where all fluid variables only weakly change in the shock front. In contrast, slow MHD shocks are driven by gas pressure where neutral species collide with ion species in a thin hot slab that closely resembles an ordinary gas dynamic shock.
We computed observational diagnostics for fast and slow shocks at velocities vs=2–4 km/s and preshock Hydrogen nuclei densities nH = 102-4 cm−3. We followed the abundances of molecules relevant for a simple oxygen chemistry and include cooling by CO, H2 and H2O. Estimates of intensities of 12CO rotational lines show that high-J lines, above J = 6 → 5, are more strongly excited in slow MHD shocks.
We examine integrated luminosity relations between the IR continuum and the CO rotational ladder observed for local (ultra) luminous infra-red galaxies ((U)LIRGs, LIR ≥ 1011 M⊙) and normal star forming galaxies in the context of radiation pressure regulated star formation proposed by Andrews & Thompson (2011). This can account for the normalization and linear slopes of the luminosity relations (log LIR = α log L'CO + β) of both low- and high-J CO lines observed for normal galaxies. Super-linear slopes occur for galaxy samples with significantly different dense gas fractions. Local (U)LIRGs are observed to have sub-linear high-J (Jup > 6) slopes or, equivalently, increasing LCOhigh-J/LIR with LIR. In the extreme ISM conditions of local (U)LIRGs, the high-J CO lines no longer trace individual hot spots of star formation (which gave rise to the linear slopes for normal galaxies) but a more widespread warm and dense gas phase mechanically heated by powerful supernovae-driven turbulence and shocks.
Single-dish observations in CS(J=7-6) using the Atacama Submillimeter Telescope Experiment (ASTE) reveal emission extending out to thousands of AU from low-mass protostars, much larger than is expected based on simple models for their envelopes. Hypotheses for this emission invoke gas dispersed from the envelope surfaces facing the bipolar outflow cavities. Here, we combine interferometric data from the Submillimeter Array (SMA) with the previous single-dish data from ASTE for the low-mass protostar L483 to study the spatial-kinematic structure of its CS(J=7-6) emission on projected scales ≳600 AU. In addition to providing more detailed information for the extended component, our combined maps reveal a compact central component in CS(J=7-6) having a steeper velocity gradient. Both the compact and extended components exhibit a velocity gradient in the opposite sense to that of a bipolar molecular outflow traced in CO(J=2-1). Finding that previous models make a number of wrong predictions for the observed features, we propose that both CS(J=7-6) components are produced by rotating and infalling gas along the envelope surfaces exposed by the bipolar outflow and therefore subjected to stellar irradiation and outflow compression.
The Planck satellite has mapped the polarized microwave sky (from 30 GHz to 353 GHz) with unprecedented sensitivity and angular resolution. This wealth of data yields the first complete map of polarized thermal emission from dust in our own Galaxy, shedding new light on the formation of dense cold structures within which new stars and planetary systems are born, under the combined effects of gravity, turbulence and magnetic fields. We present a statistical analysis of this polarized emission from nearby molecular clouds, with an emphasis on the evolution of the maximum polarization fraction observed as a function of column density, and on the anti-correlation between the polarization fraction and the local dispersion of polarization angles. To interpret this data, numerical simulations of anisotropic MHD turbulence underline the essential role played by the topology of the interstellar magnetic field, in particular its large-scale component. As an extension to this work published in Planck Intermediate Results XX (A&A, 576, 105, 2015), the statistical properties of the random component of the interstellar magnetic field are explored using a toy model based on fractional Brownian motion (fBm) fields.
Submillimetre observations of externally irradiated low-mass protostellar envelopes show that the gas temperature in the envelopes is dominated by the external irradiation. Detailed studies of the protostar IRS7B in Corona Australis also show that the chemistry is strongly affected by the irradiation, depleting the abundances of complex organic molecules.