Physical Conditions In Star-forming Regions Research Articles

Chemistry on Rotating Grain Surfaces: Ro-thermal Desorption of Molecules from Ice Mantles

Abstract It is widely believed that water and complex organic molecules (COMs) first form in the ice mantle of dust grains and are subsequently returned into the gas due to grain heating by intense radiation of protostars. Previous research on the desorption of molecules from the ice mantle assumed that grains are at rest, which is contrary to the fact that grains are suprathermally rotating as a result of their interaction with an anisotropic radiation or gas flow. To clearly understand how molecules are released into the gas phase, the effect of grain suprathermal rotation on surface chemistry must be quantified. In this paper, we study the effect of suprathermal rotation of dust grains spun-up by radiative torques on the desorption of molecules from icy grain mantles around protostars. We show that centrifugal potential energy due to grain rotation reduces the potential barrier of molecules and significantly enhances their desorption rate. We term this mechanism rotational-thermal or ro-thermal desorption. We apply the ro-thermal mechanism for studying the desorption of molecules from icy grains that are simultaneously heated to high temperatures and spun-up to suprathermal rotation by an intense radiation of protostars. We find that ro-thermal desorption is much more efficient than thermal desorption for molecules with high binding energy such as water and COMs. Our results have important implications for understanding the origin of COMs detected in star-forming regions and call for attention to the effect of suprathermal rotation of icy grains to use molecules as a tracer of physical conditions of star-forming regions.

The ALMA-PILS survey: propyne (CH3CCH) in IRAS 16293–2422

Context. Propyne (CH3CCH), also known as methyl acetylene, has been detected in a variety of environments, from Galactic star-forming regions to extragalactic sources. These molecules are excellent tracers of the physical conditions in star-forming regions, allowing the temperature and density conditions surrounding a forming star to be determined. Aims. This study explores the emission of CH3CCH in the low-mass protostellar binary, IRAS 16293–2422, and examines the spatial scales traced by this molecule, as well as its formation and destruction pathways. Methods. Atacama Large Millimeter/submillimeter Array (ALMA) observations from the Protostellar Interferometric Line Survey (PILS) were used to determine the abundances and excitation temperatures of CH3CCH towards both protostars. This data allows us to explore spatial scales from 70 to 2400 au. This data is also compared with the three-phase chemical kinetics model MAGICKAL, to explore the chemical reactions of this molecule. Results. CH3CCH is detected towards both IRAS 16293A and IRAS 16293B, and is found the hot corino components, one around each source, in the PILS dataset. Eighteen transitions above 3σ are detected, enabling robust excitation temperatures and column densities to be determined in each source. In IRAS 16293A, an excitation temperature of 90 K and a column density of 7.8 × 1015 cm−2 best fits the spectra. In IRAS 16293B, an excitation temperature of 100 K and 6.8 × 1015 cm−2 best fits the spectra. The chemical modelling finds that in order to reproduce the observed abundances, both gas-phase and grain-surface reactions are needed. The gas-phase reactions are particularly sensitive to the temperature at which CH4 desorbs from the grains. Conclusions. CH3CCH is a molecule whose brightness and abundance in many different regions can be utilised to provide a benchmark of molecular variation with the physical properties of star-forming regions. It is essential when making such comparisons, that the abundances are determined with a good understanding of the spatial scale of the emitting region, to ensure that accurate abundances are derived.

The ALMA-PILS survey: complex nitriles towards IRAS 16293–2422

Context. Complex organic molecules are readily detected in the inner regions of the gaseous envelopes of forming protostars. Their detection is crucial to understanding the chemical evolution of the Universe and exploring the link between the early stages of star formation and the formation of solar system bodies, where complex organic molecules have been found in abundance. In particular, molecules that contain nitrogen are interesting due to the role nitrogen plays in the development of life and the compact scales such molecules have been found to trace around forming protostars. Aims. The goal of this work is to determine the inventory of one family of nitrogen-bearing organic molecules, complex nitriles (molecules with a –C≡N functional group) towards two hot corino sources in the low-mass protostellar binary IRAS 16293–2422. This work explores the abundance differences between the two sources, the isotopic ratios, and the spatial extent derived from molecules containing the nitrile functional group. Methods. Using data from the Protostellar Interferometric Line Survey (PILS) obtained with ALMA, we determine abundances and excitation temperatures for the detected nitriles. We also present a new method for determining the spatial structure of sources with high line density and large velocity gradients – Velocity-corrected INtegrated emission (VINE) maps. Results. We detect methyl cyanide (CH3CN) as well as five of its isotopologues, including CHD2CN, which is the first detection in the interstellar medium (ISM). We also detect ethyl cyanide (C2H5CN), vinyl cyanide (C2H3CN), and cyanoacetylene (HC3N). We find that abundances are similar between IRAS 16293A and IRAS 16293B on small scales except for vinyl cyanide which is only detected towards the latter source. This suggests an important difference between the sources either in their evolutionary stage or warm-up timescales. We also detect a spatially double-peaked emission for the first time in molecular emission in the A source, suggesting that this source is showing structure related to a rotating toroid of material. Conclusions. With high-resolution observations, we have been able to show for the first time a number of important similarities and differences in the nitrile chemistry in these objects. These illustrate the utility of nitriles as potential tracers of the physical conditions in star-forming regions.

THE MOSDEF SURVEY: ELECTRON DENSITY AND IONIZATION PARAMETER AT z ∼ 2.3*

ABSTRACT Using observations from the MOSFIRE Deep Evolution Field survey, we investigate the physical conditions of star-forming regions in z ∼ 2.3 galaxies, specifically the electron density and ionization state. From measurements of the [O ii]λλ3726,3729 and [S ii]λλ6716,6731 doublets, we find a median electron density of ∼250 cm−3 at z ∼ 2.3, an increase of an order of magnitude compared to measurements of galaxies at z ∼ 0. While z ∼ 2.3 galaxies are offset toward significantly higher O32 values relative to local galaxies at fixed stellar mass, we find that the high-redshift sample follows a similar distribution to the low-metallicity tail of the local distribution in the O32 versus R23 and O3N2 diagrams. Based on these results, we propose that z ∼ 2.3 star-forming galaxies have the same ionization parameter as local galaxies at fixed metallicity. In combination with simple photoionization models, the position of local and z ∼ 2.3 galaxies in excitation diagrams suggests that there is no significant change in the hardness of the ionizing spectrum at fixed metallicity from z ∼ 0 to z ∼ 2.3. We find that z ∼ 2.3 galaxies show no offset compared to low-metallicity local galaxies in emission line ratio diagrams involving only lines of hydrogen, oxygen, and sulfur, but show a systematic offset in diagrams involving [N ii]λ6584. We conclude that the offset of z ∼ 2.3 galaxies from the local star-forming sequence in the [N ii] BPT diagram is primarily driven by elevated N/O at fixed O/H compared to local galaxies. These results suggest that the local gas-phase and stellar metallicity sets the ionization state of star-forming regions at z ∼ 0 and z ∼ 2.

Observations of water withHerschel/HIFI toward the high-mass protostar AFGL 2591

Water is a sensitive tracer of physical conditions in star-forming regions because of its large abundance variations between hot and cold regions. We use spectrally resolved observations of rotational lines of H$_2$O and its isotopologs with Herschel/HIFI to constrain the physical conditions of the water emitting region toward the high-mass protostar AFGL2591. We use analytical estimates and rotation diagrams to estimate T$_{ex}$ and column densities of H$_2$O of the envelope, the outflow, and a foreground cloud. Furthermore, we use the non-LTE radiative transfer code to estimate the temperature and volume density of the H$_2$O emitting gas. Assuming LTE, we estimate an T$_{ex}$ of 42 K and a column density of 2$\times$10$^{14}$ cm$^{-2}$ for the envelope and 45 K and 4$\times$10$^{13}$ cm$^{-2}$ for the outflow, in beams of 4" and 30", respectively. Non-LTE models indicate a kinetic temperature of 60-230 K and a volume density of 7$\times$10$^6$-10$^8$ cm$^{-3}$ for the envelope, and a kinetic temperature of 70-90 K and a gas density of 10$^7$-10$^8$ cm$^{-3}$ for the outflow. The o/p ratio of the foreground absorption is 1.9$\pm$0.4, suggesting a low temperature. In contrast, the o/p ratio seen in absorption by the outflow is 3.5$\pm$1.0, as expected for warm gas. The water abundance in the envelope is 10$^{-9}$, similar to the low values found for other high- and low-mass protostars, suggesting that this abundance is constant during the embedded phase of high-mass star formation. The water abundance in the outflow is 10$^{-10}$, which is 10$\times$ lower than in the envelope and in the outflows of high- and low-mass protostars. Since beam size effects can only increase this estimate by a factor of 2, we suggest that the water in the outflow is affected by dissociating UV radiation due to the low extinction in the outflow lobe.

The mass–metallicity–star formation rate relation at $\boldsymbol {z \gtrsim 2}$ with 3D Hubble Space Telescope

We present new accurate measurements of the physical properties of a statistically significant sample of 103 galaxies at z~2 using near-infrared spectroscopy taken as part of the 3D-HST survey. We derive redshifts, metallicities and star formation rates (SFRs) from the [OII], [OIII] and Hbeta nebular emission lines and exploit the multi-wavelength photometry available in CANDELS to measure stellar masses. We find the mass-metallicity relation (MZR) derived from our data to have the same trend as previous determinations in the range 0<z<3, with lower mass galaxies having lower metallicities. However we find an offset in the relation compared to the previous determination of the z~2 MZR by Erb et al. 2006b, who measure metallicities using the [NII]/Halpha ratio, with metallicities lower at a given mass. Incorporating our SFR information we find that our galaxies are offset from the Fundamental Metallicity Relation (FMR) by ~0.3 dex. We investigate the photoionization conditions and find that our galaxies are consistent with the elevated ionization parameter previously reported in high-redshift galaxies. Using the BPT diagram we argue that, if this is the case, metallicity indicators based on [NII] and Halpha may not be consistent with the ones obtained via oxygen lines and Hbeta. Using a recent determination of the theoretical evolution of the star forming sequence in the BPT diagram we convert our measured [OIII]/Hbeta line ratios to [NII]/Halpha ratios. From the [NII]/Halpha ratio we infer systematically higher metallicities in better agreement with the FMR. Our results thus suggest the evolution of the FMR previously reported at z~2-3 may be an artifact of the differential evolution in metallicity indicators, and caution against using locally calibrated metallicity relations at high redshift which do not account for evolution in the physical conditions of star-forming regions.

Physical conditions in star-forming regions around S235

Gas density and temperature in star forming regions around Sh2-235 are derived from ammonia line observations. This information is used to evaluate formation scenarios and to determine evolutionary stages of the young embedded clusters S235 East1, S235 East2, and S235 Central. We also estimate the gas mass in the embedded clusters and its ratio to the stellar mass. S235 East1 appears to be less evolved than S235 East2 and S235 Central. In S235 East1 the molecular gas mass exceeds that in the other clusters. Also, this cluster is more embedded in the parent gas cloud than the other two. Comparison with a theoretical model shows that the formation of these three clusters could have been stimulated by the expansion of the Sh2-235 HII region (hereafter S235) via a collect-and-collapse process, provided the density in the surrounding gas exceeds $3\cdot10^3$ cm$^{-3}$, or via collapse of pre-existing clumps. The expansion of S235 cannot be responsible for star formation in the southern S235 A-B region. However, formation of the massive stars in this region might have been triggered by a large-scale supernova shock. Thus, triggered star formation in the studied region may come in three varieties, namely collect-and-collapse and collapse of pre-existing clumps, both initiated by expansion of the local HII regions, and triggering by an external large-scale shock. We argue that the C235 A HII region expands into a highly non-uniform medium with increasing density. It is too young to trigger star formation in its vicinity by a collect-and-collapse process. There is an age spread inside the S235 A-B region. Massive stars in the S235 A-B region are considerably younger than lower mass stars in the same area. This follows from the estimates of their ages and the ages of associated HII regions.

Analysing derived metallicities and ionization parameters from model-based determinations in ionized gaseous nebulae

We analyze the reliability of oxygen abundances and ionization parameters obtained from different diagnostic diagrams. For this, we compiled from the literature observational emission line intensities and oxygen abundance of 446 star-forming regions whose O/H abundance was determined by direct estimation of electron temperature. The abundances compiled were compared with the values calculated in this work using different diagnostic diagrams in combination with results from a grid of photoionization models. We found that the [\ion{O}{iii}]/[\ion{O}{ii}] vs. [\ion{N}{ii}]/[\ion{O}{ii}], [\ion{O}{iii}]/H$\beta$ vs. [\ion{N}{ii}]/[\ion{O}{ii}], and ([\ion{O}{iii}]/H$\beta$)/([\ion{N}{ii}]/H$\alpha$) vs. [\ion{S}{ii}]/[\ion{S}{iii}] diagnostic diagrams give O/H values close to the $T_{\rm e}$-method, with differences of about 0.04 dex and dispersion of about 0.3 dex. Similar results were obtained by detailed models but with a dispersion of 0.08 dex. The origin of the dispersion found in the use of diagnostic diagrams is probably due to differences between the real N/O-O/H relation of the sample and the one assumed in the models. This is confirmed by the use of detailed models that do not have a fixed N/O-O/H relation. We found no correlation between ionization parameter and the metallicity for the objects of our sample. We conclude that the combination of two line ratio predicted by photoionization models, one sensitive to the metallicity and another sensitive to the ionization parameter, which takes into account the physical conditions of star-forming regions, gives O/H estimates close to the values derived using direct detections of electron temperatures.

Massive protostars as gamma-ray sources

<i>Context. <i/>Massive protostars have associated bipolar outflows with velocities of hundreds of km s<sup>-1<sup/>. Such outflows can produce strong shocks when they interact with the ambient medium leading to regions of nonthermal radio emission.<i>Aims. <i/>We aim at exploring under which conditions relativistic particles are accelerated at the terminal shocks of the protostellar jets and whether they can produce significant gamma-ray emission.<i>Methods. <i/>We estimate the conditions necessary for particle acceleration up to very high energies and gamma-ray production in the nonthermal hot spots of jets associated with massive protostars embedded in dense molecular clouds.<i>Results. <i/>We show that relativistic bremsstrahlung and proton-proton collisions can make molecular clouds with massive young stellar objects detectable by the <i>Fermi<i/> satellite at MeV-GeV energies and by Cherenkov telescope arrays in the GeV-TeV range.<i>Conclusions. <i/>Gamma-ray astronomy can be used to probe the physical conditions in star-forming regions and particle acceleration processes in the complex environment of massive molecular clouds.

Complex Chemistry in Star‐forming Regions: An Expanded Gas‐Grain Warm‐up Chemical Model

Gas-phase processes were long thought to be the key formation mechanisms for complex organic molecules in star-forming regions. However, recent experimental and theoretical evidence has cast doubt on the efficiency of such processes. Grain-surface chemistry is frequently invoked as a solution, but until now there have been no quantitative models taking into account both the high degree of chemical complexity and the evolving physical conditions of star-forming regions. Here, we introduce a new gas-grain chemical network, wherein a wide array of complex species may be formed by reactions involving radicals. The radicals we consider (H, OH, CO, HCO, CH3, CH3O, CH2OH, NH and NH2) are produced primarily by cosmic ray-induced photodissociation of the granular ices formed during the colder, earlier stages of evolution. The gradual warm-up of the hot core is crucial to the formation of complex molecules, allowing the more strongly-bound radicals to become mobile on grain surfaces. This type of chemistry is capable of reproducing the high degree of complexity seen in Sgr B2(N), and can explain the observed abundances and temperatures of a variety of previously detected complex organic molecules, including structural isomers. Many other complex species are predicted by this model, and several of these species may be detectable in hot cores. Differences in the chemistry of high- and low-mass star-formation are also addressed; greater chemical complexity is expected where evolution timescales are longer.

The H ii Region/PDR Connection: Self‐consistent Calculations of Physical Conditions in Star‐forming Regions

We have performed a series of calculations designed to reproduce infrared diagnostics used to determine physical conditions in star-forming regions. We self-consistently calculate the thermal and chemical structure of an H II region and photodissociation region (PDR) that are in pressure equilibrium. This differs from previous work, which used separate calculations for each gas phase. Our calculations span a wide range of stellar temperatures, gas densities, and ionization parameters. We describe improvements made to the spectral synthesis code Cloudy that made these calculations possible. These include the addition of a molecular network with ~1000 reactions involving 68 molecular species and improved treatment of the grain physics. Data from the Spitzer First Look Survey, along with other archives, are used to derive important physical characteristics of the H II region and PDR. These include stellar temperatures, electron densities, ionization parameters, UV radiation flux (G0), and PDR density. Finally, we calculate the contribution of the H II region to PDR emission line diagnostics, which allows for a more accurate determination of physical conditions in the PDR.

Methanol Proplyds and a Double Protoplanetary System in the Constellation Norma

We have carried out detailed studies of a star-forming region containing two maser sources in the constellation Norma. The sources display a complex spectral distribution of the maser lines and spatial distribution of the maser condensations. The maser condensations may have formed around objects that are hidden by dense molecular cocoons; the velocities of the maser features may represent Keplerian orbital motions. The cocoons, which radiate in thermal methanol and CS lines, correspond to the centers of mass in the maser sources + dense molecular core systems. The velocities of the CS lines or thermal methanol lines can be used to identify the locations of the centers of mass of these systems. If the maser radiation is generated in the atmospheres of protoplanets, the Norma radio source may correspond to two protoplanetary disks, each with a protostar and protoplanetary system. In this case, the masses of the protostars are approximately 13 Mand 38 M� . c � 2005Pleiades Publishing, Inc. Radio-spectroscopic observations of star-forming regions containing dense clumps of gas and dust with various molecular compounds are very promising for studies of the physical states of prestellar material. Radio lines emitted by such clumps can be related to various temporal, dynamical, and thermal evolu- tionary phases of the interstellar medium, and the parameters obtained from the observed emission of various molecules can be used to reconstruct the physical conditions in star-forming regions. Some molecules in prestellar condensates are subject to intense irradiation from neighboring, recently formed, young stars or to dynamical compression by flows of matter from neighboring stars, both of which can act to redistribute the level populations. This can result in nonequilibrium (maser) radiation by these molecules, which are virtually always found in associ- ation with protostellar objects. The spatial structures of the maser condensations shows that they are often laid out in chains or arcs, sometimes displaying char- acteristics consistent with Keplerian disks containing protoplanetary systems. Such disks are called proto- planetary disks, or proplyds (1).

Models of Gas-Grain Chemistry in Star-forming Regions

It is difficult if not impossible to explain the abundances of assorted interstellar molecules in both the gaseous and condensed phases without the use of grain chemistry. Unfortunately, the chemistry occurring on grains is not well understood because of a variety of uncertainties including the nature, size, and shape of dust particles, the binding energies of key species, the dominant mechanism of surface chemistry, and the correct mathematical treatment of surface processes. Still, intrepid astrochemists have used granular chemistry in chemical models of an assortment of sources including cold clouds, protostellar disks, and hot cores. Indeed, the dominant explanation of the saturated gas-phase molecules observed in hot cores involves grain chemistry during an earlier, low temperature phase. Although gas-grain models have elucidated major features of the chemistry, much more work remains to be accomplished before they can be used to help characterize the physical conditions in star-forming regions and their temporal variations.

OH masers at 1612 and 1720 MHz in star-forming regions

Masers at the ground-state OH satellite transitions near 1612 and 1720 MHz are occasionally found in star-forming regions, accompanying the dominant maser of OH at 1665 MHz. The satellite lines can then be valuable diagnostics of physical conditions in star-forming regions if we can first ascertain that all maser species truly arise from the same site. For this purpose, newly measured satellite line positions with subarcsecond accuracy are reported here, and compared with masers of main-line OH at 1665 MHz, with methanol masers at 6668 MHz, and with ultracompact H ii regions. We confirm that most of the satellite-line OH masers that we have measured are associated with star-forming regions, but a few are not: several 1612-MHz masers are associated with late-type stars, and one 1720-MHz maser is associated with a supernova remnant. The 1720-MHz masers in star-forming regions are accounted for by a pumping scheme requiring high densities, and are distinctly different from those in supernova remnants where the favoured pumping scheme operates at much lower densities.

OH Masers as Diagnostics of Physical Conditions in Star-forming Regions. II. Effects of Large Velocity Gradients and Infrared Radiation

We have studied the pumping of OH masers in star-forming regions by introducing four pumping mechanisms one at a time. The four mechanisms are collisions, local line overlap, nonlocal line overlap, and external infrared radiation field. The first two have been investigated in a previous paper (Paper I). Here we study the effects of nonlocal line overlap and of an external infrared radiation field. As in Paper I, we have used the most accurate collision rates of OH with ortho- and para-H<SUB>2</SUB>, which became available recently, and we have taken the effects of line overlap into account as accurately as possible using a recently developed formalism. We have found that, despite the significant change in the collision rates, the effects of nonlocal line overlap and of an external infrared radiation field are necessary for the explanation of the main lines in the ground state of OH. The same is true for the explanation of the general qualitative behavior of H II/OH masers. Our calculations have revealed a number of diagnostics of the physical conditions in star-forming regions. These diagnostics are enumerated in our conclusions.

OH Masers as Diagnostics of Physical Conditions in Star-forming Regions. I. Thermal Effects

OH Masers as Diagnostics of Physical Conditions in Star-forming Regions. I. Thermal Effects

Collisional pumping of H2O masers in star-forming regions

view Abstract Citations (33) References (32) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Collisional Pumping of H 2O Masers in Star-forming Regions Kylafis, Nikolaos D. ; Norman, Colin A. Abstract The pumping of H2O masers in star-forming regions by collisions with H2 molecules has been studied. It is found that the strong water masers in star-forming regions may not be explained by collisional pumping, unless very large aspect ratios (length/diameter) for the cylinder or small velocity gradients along the axis of the cylinder are assumed. In addition to the 22 GHz water maser, results are presented for other maser transitions of water in the submillimeter range. The detection of these masers with submillimeter telescopes could significantly enhance the understanding of the physical conditions in star-forming regions. Publication: The Astrophysical Journal Pub Date: June 1991 DOI: 10.1086/170071 Bibcode: 1991ApJ...373..525K Keywords: Interstellar Matter; Lasing; Radiative Transfer; Star Formation; Water Masers; Aspect Ratio; Cylinders; Hydrogen; Photons; Submillimeter Waves; Astrophysics; INTERSTELLAR: MOLECULES; MASERS; RADIATIVE TRANSFER; STARS: FORMATION full text sources ADS |