Probe Of Physical Conditions Research Articles

APEX and NOEMA observations of H2S in nearby luminous galaxies and the ULIRG Mrk 231

Context. In order to understand the evolution and feedback of active galactic nuclei (AGN) and star formation, it is important to use molecular lines as probes of physical conditions and chemistry. Aims. We use H2S to investigate the impact of starburst and AGN activity on the chemistry of the molecular interstellar medium in luminous infrared galaxies. Specifically, our aim is to search for evidence of shock enhancement of H2S related to galactic-scale mechanical feedback processes such as outflows. Methods. Using the APEX single-dish telescope, we have observed the 110–101 transition of ortho-H2S at 168 GHz towards the centres of 12 nearby luminous infrared galaxies. We have also observed the same line towards the ultra-luminous infrared galaxy Mrk 231 with the NOEMA interferometer. Results. We detected H2S towards NGC 253, NGC 1068, NGC 3256, NGC 4418, NGC 4826, NGC 4945, Circinus, M 83, and Mrk 231. Upper limits were obtained for NGC 1097, NGC 1377, and IC 860. We also detected line emission from HCN 2–1 in all galaxies in the APEX survey as well as HCO+, HNC, CH3CN, CH3OH, H2CS, HOC+, and SO in several of the sample galaxies. Mrk 231 has a rich 2 mm molecular spectrum and, in addition to H2S, we detect emission from HC3N, CH3OH, HC18O+, C2S, and CH3CCH. Four galaxies show elevated H2S emission relative to HCN: Circinus, NGC 3256, NGC 4826, and NGC 4418. We suggest that the high line ratios are caused by elevated H2S abundances in the dense gas. However, we do not find any clear connection between the H2S/HCN line intensity ratio and the presence (or speed) of molecular outflows in the sample galaxies. Therefore, H2S abundances do not seem to be globally affected by the large-scale outflows. In addition, the H2S/HCN line ratio is not enhanced in the line wings compared to the line core in Mrk 231. This suggests that H2S abundances do not increase in the dense gas in the outflow. However, we do find that the H2S and HCN luminosities (LH2S and LHCN) correlate well with the total molecular gas mass in the outflow, Moutflow(H2), in contrast to LCO and LHCO+. We also find that the line luminosity of H2S correlates with the total infrared luminosity in a similar way as that of H2O. Conclusions. We do not find any evidence of H2S abundance enhancements in the dense gas due to galactic-scale outflows in our sample galaxies, nor in the high-resolution study of Mrk 231. We discuss possible mechanisms behind the suggested H2S abundance enhancements in NGC 4418, Circinus, NGC 3256, and NGC 4826. These include radiative processes (for example X-rays or cosmic rays) or smaller-scale shocks. Further high-resolution and multi-transition studies are required to determine the cause behind the elevated H2S emission in these galaxies. We suggest that LH2S serves as a tracer of the dense gas content, similar to LHCN, and that the correlation between LH2S and Moutflow(H2) implies a relation between the dense gas reservoir and the properties and evolution of the molecular feedback. This potential link requires further study since it holds important keys to our understanding of how the properties of molecular outflows relate to those of their host galaxies. Finally, the similar infrared-correlation coefficients between H2S and H2O may indicate that they originate in the same regions in the galaxy: warm gas in shocks or irradiated by star formation or an AGN.

SRG/eROSITA discovery of a large circular SNR candidate G116.6−26.1: SN Ia explosion probing the gas of the Milky Way halo?

ABSTRACT We report a discovery of a new X-ray-selected supernova remnant (SNR) candidate SRGe J0023+3625 = G116.6−26.1 found in the SRG/eROSITA all-sky survey. The source features a large angular extent (∼4° in diameter), nearly circular shape, and X-ray spectrum dominated by emission lines of helium- and hydrogen-like oxygen. It lacks bright counterparts of similar extent at other wavelengths which could be unequivocally associated with it. Given the relatively high Galactic latitude of the source, b ≈ −26°, we interpret these observational properties as an indication of the off-disc location of this SNR candidate. Namely, we propose that this object originated from a Type Ia supernotva which exploded some 40 000 yr ago in the low density ($\sim 10^{-3}\, {\rm cm^{-3}}$) and hot ($\sim (1{-}2)\times 10^6\, {\rm K}$) gas of the Milky Way halo at a distance of ∼3 kpc from the Sun. The low density of the halo gas implies that the cooling and collisional ionization equilibrium (CEI) time-scales downstream of the forward shock are much longer than the age of the SNR. This results in a relatively soft spectrum, reflecting pre-shock ionization state of the gas, and strong boost in the plasma emissivity (compared to CEI) due to enhanced collisional excitation through the increased electron temperature. If confirmed, such a rare object would provide us with a unique ‘in situ’ probe of physical conditions (density, temperature and metallicity) near the interface between the Milky Way’s disc and the halo.

Signatures of r-process Elements in Kilonova Spectra

Abstract Binary neutron star (NS) mergers have been expected to synthesize r-process elements and emit radioactively powered radiation, called kilonovae. Although r-process nucleosynthesis was confirmed by the observations of GW170817/AT2017gfo, no trace of individual elements has been identified except for strontium. In this paper, we perform systematic calculations of line strength for bound–bound transitions and radiative transfer simulations in NS merger ejecta toward element identification in kilonova spectra. We find that Sr ii triplet lines appear in the spectrum of a lanthanide-poor model, which is consistent with the absorption feature observed in GW170817/AT2017gfo. The synthetic spectrum also shows the strong Ca ii triplet lines. This is natural because Ca and Sr are coproduced in the material with relatively high electron fraction and their ions have similar atomic structures with only one s-electron in the outermost shell. The line strength, however, highly depends on the abundance distribution and temperature in the ejecta. For our lanthanide-rich model, the spectra show the features of doubly ionized heavy elements, such as Ce, Tb, and Th. Our results suggest that the line-forming region of GW170817/AT2017gfo was lanthanide-poor. We show that the Sr ii and Ca ii lines can be used as a probe of physical conditions in NS merger ejecta. Absence of the Ca ii line features in GW170817/AT2017gfo implies that the Ca/Sr ratio is <0.002 in mass fraction, which is consistent with nucleosynthesis for electron fraction ≥0.40 and entropy per nucleon (in units of Boltzmann constant) ≥25.

Correlation between optical and UV variability of a large sample of quasars

ABSTRACT The variability of quasars across multiple wavelengths is a useful probe of physical conditions in active galactic nuclei. In particular, variable accretion rates, instabilities, and reverberation effects in the accretion disc of a supermassive black hole are expected to produce correlated flux variations in ultraviolet (UV) and optical bands. Recent work has further argued that binary quasars should exhibit strongly correlated UV and optical periodicities. Strong UV–optical correlations have indeed been established in small samples of (N ≲ 30) quasars with well-sampled light curves, and have extended the ‘bluer-when-brighter’ trend previously found within the optical bands. Here, we further test the nature of quasar variability by examining the observed-frame UV–optical correlations among bright quasars extracted from the Half Million Quasars (HMQ) catalogue. We identified a large sample of 1315 quasars in HMQ with overlapping UV and optical light curves from the Galaxy Evolution Explorer and the Catalina Real-time Transient Survey, respectively. We find that strong correlations exist in this much larger sample, but we rule out, at ∼95 per cent confidence, the simple hypothesis that the intrinsic UV and optical variations of all quasars are fully correlated. Our results therefore imply the existence of physical mechanism(s) that can generate uncorrelated optical and UV flux variations.

Quantum Behavior of Spin-Orbit Inelastic Scattering of C-Atoms by D2 at Low Energy.

Fine-structure populations and collision–induced energy transfer in atoms are of interest for many fields, from combustion to astrophysics. In particular, neutral carbon atoms are known to play a role in interstellar media, either as probes of physical conditions (ground state 3Pj spin-orbit populations), or as cooling agent (collisional excitation followed by radiative decay). This work aims at investigating the spin-orbit excitation of atomic carbon in its ground electronic state due to collisions with molecular deuterium, an isotopic variant of H2, the most abundant molecule in the interstellar medium. Spin-orbit excitations of C(3Pj) by H2 or D2 are governed by non-adiabatic and spin-orbit couplings, which make the theoretical treatment challenging, since the Born-Oppenheimer approximation no longer holds. Inelastic collisional cross-sections were determined for the C(3P0) + D2 → C(3Pj) + D2 (with j = 1 and 2) excitation process. Experimental data were acquired in a crossed beam experiment at low collision energies, down to the excitation thresholds (at 16.42 and 43.41 cm−1, respectively). C-atoms were produced mainly in their ground spin-orbit state, 3P0, by dissociation of CO in a dielectric discharge through an Even-Lavie pulsed valve. The C-atom beam was crossed with a D2 beam from a second valve. The state-to-state cross-sections were derived from the C(3Pj) (j = 1 or 2) signal measured as a function of the beam crossing angle, i.e., as a function of the collision energy. The results show different quantum behaviors for excitation to C(3P1) or C(3P2) when C(3P0) collides with ortho-D2 or normal-D2. These experimental results are analyzed and discussed in the light of highly accurate quantum calculations. A good agreement between experimental and theoretical results is found. The present data are compared with those obtained for the C-He and C-H2 collisional systems to get new insights into the dynamics of collision induced spin-orbit excitation/relaxation of atomic carbon.

Sub-millimeter heterodyne focal-plane arrays for high-resolution astronomical spectroscopy

Spectral lines are vital tools for astronomy, particularly for studying the interstellar medium, which is widely distributed throughout the volume of our Milky Way and of other galaxies. Broadband emissions, including synchrotron, free-free, and thermal dust emissions give astronomers important information. However, they do not give information about the motions of, for example, interstellar clouds, the filamentary structures found within them, star-forming dense cores, and photon-dominated regions energized by massive young stars. For study of the interstellar medium, spectral lines at sub-millimeter wavelengths are particularly important, for two reasons. First, they offer the unique ability to observe a variety of important molecules, atoms, and ions, which are the most important gas coolants (fine-structure lines of ionized and neutral carbon, neutral oxygen), probes of physical conditions (high-J transitions of CO, HF, fine-structure lines of ionized nitrogen), and of obvious biogenic importance (H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O). In addition, high-resolution observations of spectral lines offer the unique ability to disentangle the complex motions within these regions and, in some cases, to determine their arrangement along the line of sight. To accomplish this, spectral resolution high enough to resolve the spectral lines of interest is required. We can measure the resolution of the spectrometer in terms of its resolution, R = f/δf, where f is the rest frequency of the line, and δJ is the frequency resolution of the spectrometer. More-active sources can be advantageously studied with R = 3 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> , while more quiescent sources require R as high as 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> .

Herschelobservations of EXtra-Ordinary Sources (HEXOS): Methanol as a probe of physical conditions in Orion KL

We have examined methanol emission from Orion KL with of the {\em Herschel}/HIFI instrument, and detected two methanol bands centered at 524 GHz and 1061 GHz. The 524 GHz methanol band (observed in HIFI band 1a) is dominated by the isolated $\Delta$J$=$0, K$=-4\rightarrow$-3, v$_t$$=$0 Q branch, and includes 25 E-type and 2 A-type transitions. The 1061 GHz methanol band (observed in HIFI band 4b) is dominated by the $\Delta$J$=$0, K$=7\rightarrow$6, v$_t$$=$0 Q branch transitions which are mostly blended. We have used the isolated E-type v$_t$$=$0 methanol transitions to explore the physical conditions in the molecular gas. With HIFI's high velocity resolution, the methanol emission contributed by different spatial components along the line of sight toward Orion KL (hot core, low velocity flow, and compact ridge) can be distinguished and studied separately. The isolated transitions detected in these bands cover a broad energy range (upper state energy ranging from 80 K to 900 K), which provides a unique probe of the thermal structure in each spatial component. The observations further show that the compact ridge is externally heated. These observations demonstrate the power of methanol lines as probes of the physical conditions in warm regions in close proximity to young stars.

Emission line profiles as a probe of physical conditions in planetary nebulae

We present an analysis of physical conditions in planetary nebulae (PNe) in terms of collisionally-excited line (CEL) and optical-recombination line (ORL) profiles. We aim to investigate whether line profiles could be used to study the long-standing CEL/ORL abundance-discrepancy problem in nebular astrophysics. Using 1D photoionization models and their assumed velocity fields, we simulate the line profiles of various ionic species. We attempt to use our model to account for the observed CEL and ORL profiles. As a case study we present a detailed study of line profiles of the low-excitation planetary nebula (PN) IC 418. Our results show that the profiles of classical temperature and density diagnostic lines, such as [O III] 4363,5007, [S II] 6716,6731, and [Ar IV] 4711,4740, provide a powerful tool to study nebular temperature and density variations. The method enables the CEL/ORL abundance-discrepancy problem to be studied more rigorously than before. A pure photoionization model of a chemically-homogeneous nebula seems to explain the observed disagreements in the profiles for the [O III] 4363 and the 5007 lines, but cannot account for the differences between the [O III] CELs and the O II ORLs. We also investigate the temperature and density variations in the velocity space of a sample of PNe, which are found to be insignificant.

Depletion and low gas temperature in the L183 (=L134N) prestellar core: the N2H+–N2D+ tool

Context. The study of pre-stellar cores (PSCs) suffers from a lack of undepleted species to trace the gas physical properties in their very dense inner parts. Aims. We want to carry out detailed modelling of N2H+ and N2D+ cuts across the L183 main core to evaluate the depletion of these species and their usefulness as a probe of physical conditions in PSCs. Methods. We have developed a non-LTE (NLTE) Monte-Carlo code treating the 1D radiative transfer of both N2H+ and N2D+ , making use of recently published collisional coefficients with He between individual hyperfine levels. The code includes line overlap between hyperfine transitions. An extensive set of core models is calculated and compared with observations. Special attention is paid to the issue of source coupling to the antenna beam. Results. The best fitting models indicate that i) gas in the core center is very cold (7± 1 K) and thermalized with dust, ii) depletion of N2H+ does occur, starting at densities 5–7E5 cm−3 and reaching a factor of 6 (+13/−3) in abundance, iii) deuterium fractionation reaches ∼70% at the core center, and iv) the density profile is proportional to r^-1 out to ∼4000 AU, and to r^−2 beyond. Conclusions. Our NLTE code could be used to (re-)interpret recent and upcoming observations of N2H+ and N2D+ in many pre-stellar cores of interest, to obtain better temperature and abundance profiles.

Faster than light: superluminal motion and light echoes

Apparent motions at speeds exceeding that of light occur in several different types of astronomical object. They are straightforward to understand as a consequence of the finite velocity of light, and can be used as probes of physical conditions. Two examples are described: superluminal motion in extragalactic radio sources and light echoes around supernovae.

Formaldehyde as a probe of physical conditions in dense molecular clouds

view Abstract Citations (191) References (112) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Formaldehyde as a Probe of Physical Conditions in Dense Molecular Clouds Mangum, Jeffrey G. ; Wootten, Alwyn Abstract We present a detailed analysis describing the utility of the formaldehyde (H2CO) molecule in the derivation of the kinetic temperature and spatial density within molecular clouds. Measurements of 13 transitions from both the ortho and para species of H2CO have been made toward a sample of 11 active star formation regions. These H2CO transitions range in frequency from 211 to 365 GHz and in upper-state energy from 21 to 241 K. This range in excitation has allowed us to analyze H2CO sensitivity to both cool (TK less than or approximately equal to 50 K) and warm (TK greater than 50 K) molecular material. Using a spherical large velocity gradient (LVG) model to solve for the excitation of H2CO, we analyze the sensitivity of several ortho- and para-H2CO transition intensity ratios to the kinetic temperature and spatial density within molecular clouds. Using our measured H2CO radiation temperatures, we have constrained LVG model solutions for the kinetic temperature, spatial density, and H2CO species column density in each of the sources in our sample. In all of the regions in our sample, though, we measure kinetic temperatures greater than 50 K, significantly higher than previous estimates for many of these sources. Using our ortho- and para-H2CO measurements we have calculated the N(ortho-H2CO)/N(para-H2CO) ratio in several of the sources in our sample. Our measurements indicate that N(ortho-H2CO)/N(para-H2CO) less than 3 for most of our sources. When combined with the relatively high kinetic temperature in these objects, this N(ortho-H2CO)/N(para-H2CO) ratio suggests that dust grains might play an active role in H2CO chemistry. Publication: The Astrophysical Journal Supplement Series Pub Date: November 1993 DOI: 10.1086/191841 Bibcode: 1993ApJS...89..123M Keywords: Energy Levels; H Ii Regions; Interstellar Gas; Molecular Clouds; Molecular Spectra; Radiative Transfer; Star Formation; Velocity Distribution; Molecular Spectroscopy; Space Density; Spatial Resolution; Temperature; Astrophysics; ISM: MOLECULES; ISM: H II REGIONS; ISM: CLOUDS; MOLECULAR PROCESSES full text sources ADS | data products SIMBAD (14)

Infrared molecular spectroscopy toward the Orion IRc2 and IRc7 sources - A new probe of physical conditions and abundances in molecular clouds

view Abstract Citations (79) References (47) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Infrared Molecular Spectroscopy toward the Orion IRc2 and IRc7 Sources: A New Probe of Physical Conditions and Abundances in Molecular Clouds Evans, Neal J., II ; Lacy, J. H. ; Carr, John S. Abstract Observations of infrared absorption lines of C2H2, HCN, OCS, CO, and NH3 toward the infrared cluster in the Orion molecular cloud are presented. On the basis of the relative line strengths and the detection of several (C-13)CH2 lines, it is contended that the lines are narrow (Gaussian dispersion is about 1 km/s) and saturated. The derived temperature is about 140-150 K, and the column density of C2H2 is 1.4 x 10 exp 17/sq cm toward IRc2, with about half as much C2H2 at a similar temperature toward IRc7. Several lines were observed at 13.5 micron of the nu2 vibrational band of HCN toward IRc2 and IRc7. The observed line strengths toward both sources indicate that HCN is in LTE for rotational levels through J = 14. The temperature is about 150 K, consistent with the temperature derived from C2H2, and the column density of HCN toward IRc2 is about 2.6 x 10 exp 17/sq cm. Publication: The Astrophysical Journal Pub Date: December 1991 DOI: 10.1086/170824 Bibcode: 1991ApJ...383..674E Keywords: Abundance; Infrared Sources (Astronomy); Infrared Spectroscopy; Molecular Clouds; Molecular Spectroscopy; Absorption Spectra; Ammonia; Astronomical Models; Carbon Monoxide; Orion Constellation; Vibrational Spectra; Astrophysics; INFRARED: SOURCES; INTERSTELLAR: ABUNDANCES; INTERSTELLAR: MOLECULES; LINE IDENTIFICATIONS full text sources ADS | data products SIMBAD (3)

Asteroid compositional «rings»: clues to the compositions of primordial planetesimals in the middle solar system

ABSTRACTIt has recently been established that the distribution of asteroid taxonomic types at distances between 2.1 and 5.3 astronomical units is highly structured. There are four major, overlapping but nevertheless compositionally distinct, “rings” of asteroids present within this range of heliocentric distance. These “rings”, within which ~ 80% of each of four major taxonomic types (S, C, P, and D) fall, are centered at 2.6 (0.7), 2.9 (0.8), 3.4 (0.7), and 4.6 (1.5) AU respectively, where the numbers within parentheses are the ring “widths” in AU. The overall physical resemblence between the asteroid “rings” and planetary rings is poor; physically the asteroid belt more closely resembles a debris strewn satellite system. This structure is consistent with these objects having been formed directly from the solar nebula at, or near, the heliocentric distances at which we find them today. Once the mineralogy of these taxonomic types is firmly established, and complications arising from post-accreationary metamorphism are dealt with, they may be used as probes of physical conditions in the early solar system. In particular, the identification of primordial planetesimals will allow us to obtain a first-hand look at the siblings of the planetesimals responsible for the final stage of planetary accretion.

Atomic nitrogen as a probe of physical conditions in the interstellar medium

Atomic nitrogen as a probe of physical conditions in the interstellar medium

Interstellar molecules and the problem of laboratory data

The use of interstellar molecules as probes of physical conditions in interstellar clouds is hampered by the lack of basic laboratory data. The excitation of interstellar molecules is poorly understood because the nature of the interaction of molecules with radiation and with neutral particles is largely undknown. The mechanisms of formation and destruction of interstellar molecules are presently speculative, because little data exists in such areas as gas-phase ion-molecule reactions and exchange reactions, and reactions of various types on surfaces. Specific needs with regard to laboratory data are discussed in these and other areas.