Solis IV. Hydrocarbons in the OMC–2 Fir 4 Region, a Probe of Energetic Particle Irradiation of the Region (original) (raw)
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High Spectral Resolution SOFIA/EXES Observations of C2H2 toward Orion IRc2
The Astrophysical Journal
We present high-spectral resolution observations from 12.96-13.33 microns towards Orion IRc2 using the mid-infrared spectrograph, EXES, on SOFIA. These observations probe the physical and chemical conditions of the Orion hot core, which is sampled by a bright, compact, mid-infrared background continuum source in the region, IRc2. All ten of the rovibrational C 2 H 2 transitions expected in our spectral coverage, are detected with high S/N, yielding continuous coverage of the R-branch lines from J=9-8 to J=18-17, including both ortho and para species. Eight of these rovibrational transitions are newly reported detections. The isotopologue, 13 CCH 2 , is clearly detected with high signal-to-noise. This enabled a direct measurement of the 12 C/ 13 C isotopic ratio for the Orion hot core of 14 ± 1 and an estimated maximum value of 21. We also detected several HCN rovibrational lines. The ortho and para C 2 H 2 ladders are clearly separate and tracing two different temperatures, 226 K and 164 K, respectively, with a non-equilibrium ortho to para ratio (OPR) of 1.7 ± 0.1. Additionally, the ortho and para V LSR values differ by about 1.8 ± 0.2 km s −1 , while, the mean line widths differ by 0.7 ± 0.2 km s −1 , suggesting that these species are not uniformly mixed along the line of sight to IRc2. We propose that the abnormally low C 2 H 2 OPR could be a remnant from an earlier, colder phase, before the density enhancement (now the hot core) was impacted by shocks generated from an explosive event 500 yrs ago.
SOLIS II. Carbon-chain growth in the Solar-type protocluster OMC2-FIR4
arXiv (Cornell University), 2017
The interstellar delivery of carbon atoms locked into molecules might be one of the key ingredients for the emergence of life. Cyanopolyynes are carbon chains delimited at their two extremities by an atom of hydrogen and a cyano group, meaning that they could be excellent reservoirs of carbon. The simplest member, HC 3 N, is ubiquitous in the galactic interstellar medium and found also in external galaxies. Thus, understanding the growth of cyanopolyynes in regions forming stars similar to our Sun, and what affects them, is particularly relevant. In the framework of the IRAM/NOEMA Large Program SOLIS (Seeds Of Life In Space), we have obtained a map of two cyanopolyynes, HC 3 N and HC 5 N, in the protocluster OMC-2 FIR4. Because our Sun is thought to be born in a rich cluster, OMC-2 FIR4 is one of the closest and best known representatives of the environment in which the Sun may have been born. We find a HC 3 N/HC 5 N abundance ratio across the source in the range ∼1−30, with the smallest values (≤10) in FIR5 and in the eastern region of FIR4. The ratios ≤10 can be reproduced by chemical models only if: (1) the cosmic-ray ionisation rate ζ is ∼4 × 10 −14 s −1 ; (2) the gaseous elemental ratio C/O is close to unity; and (3) oxygen and carbon are largely depleted. The large ζ is comparable to that measured in FIR4 by previous works and was interpreted as due to a flux of energetic (≥10 MeV) particles from embedded sources. We suggest that these sources could lie east of FIR4 and FIR5. A temperature gradient across FIR4, with T decreasing from east to west by about 10 K, could also explain the observed change in the HC 3 N/HC 5 N line ratio, without the need of a cosmic ray ionisation rate gradient. However, even in this case, a high constant cosmic-ray ionisation rate (of the order of 10 −14 s −1) is necessary to reproduce the observations.
The high-resolution far-infrared spectrum of methane at the SOLEIL synchrotron
Journal of Quantitative Spectroscopy and Radiative Transfer, 2010
As a tetrahedral molecule, methane has no permanent dipole moment. Its spectrum, however, displays faint absorption lines in the THz region, due to centrifugal distorsion effects. This is important for planetary applications since this region is used to measure methane concentration in some planetary atmospheres, in particular on Titan. Up to now, all measurements relied either on some old low resolution infrared absorption spectra, or on high resolution Stark measurements for low J values only. Even if these results have been reexamined recently [E. H. Wishnow, G. S. Orton, I. Ozier and H. P. Gush, J. Quant. Spectrosc. Radiat. Transfer 103, 102-117 (2007)], it seemed highly desirable to obtain much more precise laboratory data.
[C II] emission from L1630 in the Orion B molecular cloud
Astronomy & Astrophysics
Context. L1630 in the Orion B molecular cloud, which includes the iconic Horsehead Nebula, illuminated by the star system σ Ori, is an example of a photodissociation region (PDR). In PDRs, stellar radiation impinges on the surface of dense material, often a molecular cloud, thereby inducing a complex network of chemical reactions and physical processes. Aims. Observations toward L1630 allow us to study the interplay between stellar radiation and a molecular cloud under relatively benign conditions, that is, intermediate densities and an intermediate UV radiation field. Contrary to the well-studied Orion Molecular Cloud 1 (OMC1), which hosts much harsher conditions, L1630 has little star formation. Our goal is to relate the [C ii] fine-structure line emission to the physical conditions predominant in L1630 and compare it to studies of OMC1. Methods. The [C ii] 158 µm line emission of L1630 around the Horsehead Nebula, an area of 12 × 17 , was observed using the upgraded German Receiver for Astronomy at Terahertz Frequencies (upGREAT) onboard the Stratospheric Observatory for Infrared Astronomy (SOFIA). Results. Of the [C ii] emission from the mapped area 95%, 13 L , originates from the molecular cloud; the adjacent H ii region contributes only 5%, that is, 1 L. From comparison with other data (CO (1−0)-line emission, far-infrared (FIR) continuum studies, emission from polycyclic aromatic hydrocarbons (PAHs)), we infer a gas density of the molecular cloud of n H ∼ 3 × 10 3 cm −3 , with surface layers, including the Horsehead Nebula, having a density of up to n H ∼ 4 × 10 4 cm −3. The temperature of the surface gas is T ∼ 100 K. The average [C ii] cooling efficiency within the molecular cloud is 1.3 × 10 −2. The fraction of the mass of the molecular cloud within the studied area that is traced by [C ii] is only 8%. Our PDR models are able to reproduce the FIR-[C ii] correlations and also the CO (1−0)-[C ii] correlations. Finally, we compare our results on the heating efficiency of the gas with theoretical studies of photoelectric heating by PAHs, clusters of PAHs, and very small grains, and find the heating efficiency to be lower than theoretically predicted, a continuation of the trend set by other observations. Conclusions. In L1630 only a small fraction of the gas mass is traced by [C ii]. Most of the [C ii] emission in the mapped area stems from PDR surfaces. The layered edge-on structure of the molecular cloud and limitations in spatial resolution put constraints on our ability to relate different tracers to each other and to the physical conditions. From our study, we conclude that the relation between [C ii] emission and physical conditions is likely to be more complicated than often assumed. The theoretical heating efficiency is higher than the one we calculate from the observed [C ii] emission in the L1630 molecular cloud.
The TEXES Survey for H 2 Emission from Protoplanetary Disks
The Astrophysical Journal, 2008
We report the results of a search for pure rotational molecular hydrogen emission from the circumstellar environments of young stellar objects with disks using the Texas Echelon Cross Echelle Spectrograph (TEXES) on the NASA Infrared Telescope Facility and the Gemini North Observatory. We searched for mid-infrared H 2 emission in the S(1), S(2), and S(4) transitions. Keck/NIRSPEC observations of the H 2 S(9) transition were included for some sources as an additional constraint on the gas temperature. We detected H 2 emission from 6 of 29 sources observed: AB Aur, DoAr 21, Elias 29, GSS 30 IRS 1, GV Tau N, and HL Tau. Four of the six targets with detected emission are class I sources that show evidence for surrounding material in an envelope in addition to a circumstellar disk. In these cases, we show that accretion shock heating is a plausible excitation mechanism. The detected emission lines are narrow (∼10 km s −1), centered at the stellar velocity, and spatially unresolved at scales of 0.4 ′′ , which is consistent with origin from a disk at radii 10-50 AU from the star. In cases where we detect multiple emission lines, we derive temperatures 500 K from ∼1 M ⊕ of gas. Our upper limits for the non-detections place upper limits on the amount of H 2 gas with T >500 K of less than a few Earth masses. Such warm gas temperatures are significantly higher than the equilibrium dust temperatures at these radii, suggesting that the gas is decoupled from the dust in the regions we are studying and that processes such as UV, X-ray, and accretion heating may be important.
The Astrophysical Journal, 2008
Using the ASTE 10 m submillimeter telescope and the 1.4 m Infrared Survey Facility (IRSF), we performed an extensive outflow survey in the Orion Molecular Cloud -2 and -3 region. Our survey, which includes 41 potential star-forming sites, has been newly compiled using multi-wavelength data based on millimeter-and submillimeter-continuum observations as well as radio continuum observations. From the CO (3-2) observations performed with the ASTE 10 m telescope, we detected 14 CO molecular outflows, seven of which were newly identified. This higher detection rate, as compared to previous CO (1-0) results in the same region, suggests that CO (3-2) may be a better outflow tracer. Physical properties of these outflows and their possible driving sources were derived. Derived parameters were compared with those of CO outflows in low-and high-mass starforming regions. We show that the CO outflow momentum correlates with the bolometric luminosity of the driving source and with the envelope mass, regardless of the mass of the driving sources. In addition to these CO outflows, seven sources having NIR features suggestive of outflows were also identified.
Laboratory Studies on the Irradiation of Methane in Interstellar, Cometary, and Solar System Ices
Astrophysical Journal, 2006
Pure methane ices (CH 4 ) were irradiated at 10 K with energetic electrons to mimic the energy transfer processes that occur in the track of the trajectories of MeV cosmic-ray particles. The experiments were monitored via an FTIR spectrometer (solid state) and a quadrupole mass spectrometer (gas phase). Combined with electronic structure calculations, this paper focuses on the identification of CH x (x ¼ 1Y4) and C 2 H x (x ¼ 2Y6) species and also investigates their formation pathways quantitatively. The primary reaction step is determined to be the cleavage of a carbonhydrogen bond of the methane molecule to form a methyl radical (CH 3 ) plus a hydrogen atom. Hydrogen atoms recombined to form molecular hydrogen, the sole species detected in the gas phase during the irradiation exposure. In the matrix two neighboring methyl radicals can recombine to form an internally excited ethane molecule (C 2 H 6 ), which either can be stabilized by the surrounding matrix or was found to decompose unimolecularly to the ethyl radical (C 2 H 5 ) plus atomic hydrogen and then to the ethylene molecule (C 2 H 4 ) plus molecular hydrogen. The initially synthesized ethane, ethyl, and ethylene molecules can be radiolyzed subsequently by the impinging electrons to yield the vinyl radical (C 2 H 3 ) and acetylene (C 2 H 2 ) as degradation products. Upon warming the ice sample after the irradiation, the new species are released into the gas phase, simulating the sublimation processes interstellar ices undergo during the hot core phase or comets approaching perihelion. Our investigations also aid the understanding of the synthesis of hydrocarbons likely to be formed in the aerosol particles and organic haze layers of hydrocarbon-rich atmospheres of planets and their moons such as Titan.
Substantial reservoirs of molecular hydrogen in the debris disks around young stars
Nature, 2001
magnitude larger than in star-forming galaxies. This all suggests a heating source other than stars and the AGN is the obvious alternative. Differential magni®cation of AGN-heated gas therefore seems to be responsible for the high luminosity of the CO9 ! 8 line. Although highly excited regions with CO emission ampli®ed by gravitational lensing are good markers of the presence of molecular gas at high redshifts, they may give a poor representation of its average physical conditions, particularly of its total mass and distribution. At z. 3, this excitation bias becomes more serious since millimetre-wave interferometers, the instruments usually used for such searches, can detect only CO J 1 ! J; J. 2 whose excitation requirements are nH 2 > 10 4 cm 2 3 and T > 50 K. These values, being typical of the average conditions in star-forming clouds, mark a gradual excitation turnover. Observations of the CO1 ! 0 and CO2 ! 1 transitions, with their minimal excitation requirements, may reveal much larger molecular gas reservoirs at high redshifts. The VLA is currently the only instrument capable of sensitive, sub-arcsecond observations of these lines and will thus be an important tool for unbiased surveys of metal-enriched H 2 gas around objects in the distant Universe 21. M Methods Our observations took place during 23±24 April 2000 using the VLA in its C con®guration. After all overheads, 20 hours were spent integrating on APM08279+5255. 19 of the 27 antennas were equipped with 43-GHz receivers (T sys 45±100 K); all were equipped with 22-GHz receivers, including eleven new receivers with T sys ,40 K (compared with 100±150 K for the remainder). We used the new fast-switching technique 22,23 , recording data every 3.3 s, with 30 s on the calibrator and 170 s on the source (210 s in the 22-GHz band); the typical slewing overhead to the compact phase calibrator, 0824+558, 3.38 away was 7 s. The pointing accuracy was checked every hour. This technique yields diffraction-limited images at the highest VLA operating frequencies over long baselines. Conventional phase referencing would not have been able to track tropospheric phase variations and self-calibration techniques could not be employed because of the low signal-to-noise ratio per baseline. We used the continuum mode, placing emphasis on sensitivity rather than line pro®le information. A dual-polarization 50-MHz band was placed as close to the expected line centre as could be allowed by correlator limitations: 23.4649 GHz for CO J 1 ! 0 (+91 km s-1 from the line centre 3). Bandpass roll-off limited the effective bandwidth per intermediate frequency (IF) to ,45 MHz which, at z 3:9, corresponds to Dv,575 km s 2 1 at 23 GHz. The remaining IF pair was tuned to simultaneously observe the continuum at 23.3649 GHz (+1,280 km s-1). We obtained matching velocity coverage at 43 GHz (CO J 2 ! 1) by placing two contiguous 50-MHz dual-polarization bands at 46.9399 GHz (-20 km s-1 from the line centre). The continuum was observed on a separate occasion with both IF pairs tuned to 43.3 GHz. The¯ux-density scale was ®xed using 3C286; the uncertainty is ,15% at 23 GHz and ,20% at 46 GHz. Calibration and reduction of the data was standard in most respects and the r.m.s. noise in all the maps is similar to the expected theoretical limit. We cannot completely rule out the possibility that phase errorsÐcaused by atmospheric¯uctuations too fast to be tracked even by our fast-switching schemeÐare the cause of the extended CO emission, but we consider it to be remote. This is borne out by an extensive series of tests: inspection of the raw visibilities; separate imaging of leftand right-hand polarization maps; imaging of the most phase-stable subset of the data; and imaging of the calibrator source. The agreement between the nuclear CO J 2 ! 1 emission and its associated 3.5-cm continuum is also strong testimony to the coherence of the CO J 2 ! 1 map; the point-like 23.4-GHz continuum emission, observed simultaneously with the neighbouring line emission, provides similar supporting evidence for the resolved CO J 1 ! 0 emission.
The [ITAL]ISO[/ITAL]–SWS 2.4–45.2 Micron Spectrum toward Orion IR[CLC]c[/CLC]2
The Astrophysical Journal, 1998
The complete infrared spectrum from 2.4 to 45.2 mm toward the prototypical massive star-forming region Orion IRc2 is presented, obtained with the Short Wavelength Spectrometer (SWS) on board the Infrared Space Observatory (ISO) at a resolving power. A wealth of emission and absorption features is found, l/Dl ≈ 1300-2500 including H 2 vibration-rotation lines, the full set of H 2 pure rotational lines (0,0) S(1)-S(17), H recombination lines, ionic fine-structure lines, PAH emission features, and absorption and emission bands by interstellar ices and gas-phase molecules, including CO 2 , CH 4 , and SO 2. Particularly interesting is the detection of strong emission and absorption lines in the H 2 O n 2 bending mode at 6.2 mm and the observation of highly excited pure rotational lines of H 2 O in absorption at 25-45 mm. The origin of these lines in each of the physical components included in the ISO-SWS beam (H ii region, PDR, quiescent ridge, shocked low-velocity plateau) is briefly discussed.
Laboratory and Theoretical Simulation of 3.4 μm Spectra of Hydrocarbons in Interstellar Sources
The Astrophysical Journal, 2005
The presence of hydrocarbons in interstellar clouds and in some emission objects can be inferred from the appearance of spectral features near 3.4 m that are characteristic of CH 2 and CH 3 groups. While the 3.4 m band attributable to these hydrocarbons has been found to be similar in sources such as GC IRS 6E and CRL 618 (Chiar et al. 1998), there are significant variations in the relative amplitude of individual components in other sources such as NGC 7538 IRS9 (Allamandola et al. 1992). This indicates that the composition of these hydrocarbons may depend on ambient conditions in interstellar clouds. To investigate this possibility, we have analyzed observational IR spectra of GC IRS 6E, CRL 618, IRAS 05341+ 0852, and NGC 7538 IRS9 to extract spectral bands associated with CH 2 and CH 3 groups in each of these sources. These components are compared with the features that appear in laboratory absorption and emission spectra of hydrogenated amorphous carbon. It is found that significant differences exist in the CH 2 /CH 3 ratio in individual sources. In particular, we find that CH 3 groups are suppressed in dense cloud dust but that CH 2 groups are still abundant. The properties of individual spectral components within the 3.4 m interstellar band are discussed and compared to laboratory and theoretical data on IR spectra of certain hydrocarbon molecules, as well as that of hydrogenated amorphous carbon. Simulation of 3.4 m spectra using a random covalent network model is shown to provide a useful way to extract structural and bonding information for specific chemical groups in interstellar material.