| 1. |
- Benz, A. O., et al.
(författare)
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Water in star-forming regions with Herschel (WISH): VI. Constraints on UV and X-ray irradiation from a survey of hydrides in low- to high-mass young stellar objects
- 2016
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 590, s. Art. no. A105-
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Tidskriftsartikel (refereegranskat)abstract
- Context. Hydrides are simple compounds containing one or a few hydrogen atoms bonded to a heavier atom. They are fundamental precursor molecules in cosmic chemistry and many hydride ions have become observable in high quality for the first time thanks to the Herschel Space Observatory. Ionized hydrides such as CH+ and OH+ (and also HCO+), which affect the chemistry of molecules such as water, provide complementary information on irradiation by far-UV (FUV) or X-rays and gas temperature. Aims. We explore hydrides of the most abundant heavier elements in an observational survey covering young stellar objects (YSOs) with different mass and evolutionary state. The focus is on hydrides associated with the dense protostellar envelope and outflows, contrary to previous work that focused on hydrides in diffuse foreground clouds. Methods. Twelve YSOs were observed with HIFI on Herschel in six spectral settings providing fully velocity-resolved line profiles as part of the Water in star-forming regions with Herschel (WISH) program. The YSOs include objects of low (Class 0 and I), intermediate, and high mass, with luminosities ranging from 4 L? to 2 × 105 L?. Results. The targeted lines of CH+, OH+, H2O+, C+, and CH are detected mostly in blue-shifted absorption. H3O+ and SH+ are detected in emission and only toward some high-mass objects. The observed line parameters and correlations suggest two different origins related to gas entrained by the outflows and to the circumstellar envelope. The derived column densities correlate with bolometric luminosity and envelope mass for all molecules, best for CH, CH+, and HCO+. The column density ratios of CH+/OH+ are estimated from chemical slab models, assuming that the H2 density is given by the specific density model of each object at the beam radius. For the low-mass YSOs the observed ratio can be reproduced for an FUV flux of 2-400 times the interstellar radiation field (ISRF) at the location of the molecules. In two high-mass objects, the UV flux is 20-200 times the ISRF derived from absorption lines, and 300-600 ISRF using emission lines. Upper limits for the X-ray luminosity can be derived from H3O+ observations for some low-mass objects. Conclusions. If the FUV flux required for low-mass objects originates at the central protostar, a substantial FUV luminosity, up to 1.5 L?, is required. There is no molecular evidence for X-ray induced chemistry in the low-mass objects on the observed scales of a few 1000 AU. For high-mass regions, the FUV flux required to produce the observed molecular ratios is smaller than the unattenuated flux expected from the central object(s) at the Herschel beam radius. This is consistent with an FUV flux reduced by circumstellar extinction or by bloating of the protostar.
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| 2. |
- Indriolo, N., et al.
(författare)
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Chemical Analysis of a Diffuse Cloud along a Line of Sight toward W51: Molecular Fraction and Cosmic-Ray Ionization Rate
- 2012
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Ingår i: Astrophysical Journal. - 1538-4357 .- 0004-637X. ; 758:2, s. 83 (1-8)
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Tidskriftsartikel (refereegranskat)abstract
- Absorption lines from the molecules OH+, H2O+, and H-3(+) have been observed in a diffuse molecular cloud along a line of sight near W51 IRS2. We present the first chemical analysis that combines the information provided by all three of these species. Together, OH+ and H2O+ are used to determine the molecular hydrogen fraction in the outskirts of the observed cloud, as well as the cosmic-ray ionization rate of atomic hydrogen. H-3(+) is used to infer the cosmic-ray ionization rate of H-2 in the molecular interior of the cloud, which we find to be zeta(2) = (4.8 +/- 3.4) x 10(-16) s(-1). Combining the results from all three species we find an efficiency factor-defined as the ratio of the formation rate of OH+ to the cosmic-ray ionization rate of H-of epsilon = 0.07+/-0.04, much lower than predicted by chemical models. This is an important step in the future use of OH+ and H2O+ on their own as tracers of the cosmic-ray ionization rate.
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| 3. |
- Indriolo, N., et al.
(författare)
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Herschel Survey of Galactic OH+, H2O+, and H3O+: Probing the Molecular Hydrogen Fraction and Cosmic-Ray Ionization Rate
- 2015
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Ingår i: Astrophysical Journal. - 1538-4357 .- 0004-637X. ; 800:1, s. 40-
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Tidskriftsartikel (refereegranskat)abstract
- In diffuse interstellar clouds the chemistry that leads to the formation of the oxygen-bearing ions OH+, H2O+, and H3O+ begins with the ionization of atomic hydrogen by cosmic rays, and continues through subsequent hydrogen abstraction reactions involving H2. Given these reaction pathways, the observed abundances of these molecules are useful in constraining both the total cosmic-ray ionization rate of atomic hydrogen (ζH) and molecular hydrogen fraction (f_H_2). We present observations targeting transitions of OH+, H2O+, and H3O+ made with the Herschel Space Observatory along 20 Galactic sight lines toward bright submillimeter continuum sources. Both OH+ and H2O+ are detected in absorption in multiple velocity components along every sight line, but H3O+ is only detected along 7 sight lines. From the molecular abundances we compute f_H_2 in multiple distinct components along each line of sight, and find a Gaussian distribution with mean and standard deviation 0.042 ± 0.018. This confirms previous findings that OH+ and H2O+ primarily reside in gas with low H2 fractions. We also infer ζH throughout our sample, and find a lognormal distribution with mean log (ζH) = –15.75 (ζH = 1.78 × 10–16 s–1) and standard deviation 0.29 for gas within the Galactic disk, but outside of the Galactic center. This is in good agreement with the mean and distribution of cosmic-ray ionization rates previously inferred from H_3^+ observations. Ionization rates in the Galactic center tend to be 10-100 times larger than found in the Galactic disk, also in accord with prior studies.
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| 4. |
- Indriolo, Nick, et al.
(författare)
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High Spectral Resolution Observations toward Orion BN at 6 μm: No Evidence for Hot Water
- 2018
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 2041-8205. ; 865:2
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Tidskriftsartikel (refereegranskat)abstract
- The Becklin-Neugebauer (BN) object in Orion has a large proper motion and radial velocity with respect to the gas and other stars in the region where it is presumed to have formed. Multiple dynamical interaction scenarios have been proposed to explain this motion. In one case BN is thought to have interacted with stars in the Trapezium cluster, while in another it is thought to have interacted with source I while deeply embedded in molecular gas. If there is dense gas that has been retained in close proximity to BN, it may be evidence that the latter scenario is favored. We observed BN at high spectral resolution in three windows near 6 mu m using the Echelon-Cross-Echelle Spectrograph on board the Stratospheric Observatory for Infrared Astronomy targeting the nu(2) vibrational band of H2O. Absorption from only three transitions of H2O is detected, and through kinematic analysis is associated with cool, dense foreground gas, not BN itself. We find no evidence for H2O absorption or emission at the systemic velocity of BN.
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| 5. |
- Indriolo, Nick, et al.
(författare)
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SOFIA/EXES observations of water absorption in the protostar AFGL 2591 at high spectral resolution
- 2015
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Ingår i: Astrophysical Journal Letters. - 2041-8213. ; 802:2
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Tidskriftsartikel (refereegranskat)abstract
- We present high spectral resolution (similar to 3 km s(-1)) observations of the nu(2) ro-vibrational band of H2O in the 6.086-6.135 mu m range toward the massive protostar AFGL 2591 using the Echelon-Cross-Echelle Spectrograph (EXES) on the Stratospheric Observatory for Infrared Astronomy (SOFIA). Ten absorption features are detected in total, with seven caused by transitions in the nu(2) band of H2O, two by transitions in the first vibrationally excited nu(2) band of H2O, and one by a transition in the nu(2) band of (H2O)-O-18. Among the detected transitions is the nu(2) 1(1,1)-0(0,0) line that probes the lowest-lying rotational level of para-H2O. The stronger transitions appear to be optically thick, but reach maximum absorption at a depth of about 25%, suggesting that the background source is only partially covered by the absorbing gas or that the absorption arises within the 6 mu m emitting photosphere. Assuming a covering fraction of 25%, the H2O column density and rotational temperature that best fit the observed absorption lines are N(H2O) = (1.3 +/- 0.3) x 10(19) cm(-2) and T = 640 +/- 80 K.
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| 6. |
- Neufeld, D.A., et al.
(författare)
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HERSCHEL OBSERVATIONS of INTERSTELLAR CHLORONIUM. II. DETECTIONS TOWARD G29.96-0.02, W49N, W51, and W3(OH), and DETERMINATIONS of the ORTHO-TO-PARA and 35Cl/37Cl ISOTOPIC RATIOS
- 2015
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Ingår i: Astrophysical Journal. - 1538-4357 .- 0004-637X. ; 807:1
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Tidskriftsartikel (refereegranskat)abstract
- We report additional detections of the chloronium molecular ion, H2Cl+, toward four bright submillimeter continuum sources: G29.96-0.02, W49N, W51, and W3(OH). With the use of the HIFI instrument on board the Herschel Space Observatory, we observed the 212-101 transition of ortho-H235Cl+ at 781.627 GHz in absorption toward all four sources. Much of the detected absorption arises in diffuse foreground clouds that are unassociated with the background continuum sources and in which our best estimates of the N(H2Cl+)/N(H) ratio lie in the range (0.9-4.8)× 10-9. These chloronium abundances relative to atomic hydrogen can exceed the predictions of current astrochemical models by up to a factor of 5. Toward W49N, we have also detected the 212-101 transition of ortho-H237Cl+ at 780.053 GHz and the 111-000 transition of para-H235Cl+ at 485.418 GHz. These observations imply H235Cl+/H237Cl+ column density ratios that are consistent with the solar system 35Cl/37Cl isotopic ratio of 3.1, and chloronium ortho-to-para ratios consistent with 3, the ratio of spin statistical weights.
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| 7. |
- Schilke, P., et al.
(författare)
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Ubiquitous Argonium, ArH+, in the Diffuse Interstellar Medium
- 2014
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Ingår i: 13th International HITRAN Conference, Cambridge MA, USA.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- ArH+ is isoelectronic with HCl. The J = 1-0 and 2-1 transitions of 36ArH+ near 617.5 and 1234.6 GHz, respectively, have been identified very recently as emission lines in spectra obtained with Herschel toward the Crab Nebula supernova remnant.1 On Earth, 40Ar is by far the most abundant isotope, being almost exclusively formed by the radioactive decay of 40K. However, 36Ar is the dominant isotope in the Universe.In the course of unbiased line surveys of the massive and very luminous Galactic Center star-forming regions Sagittarius B2(M) and (N) with the high-resolution instrument HIFI on board of Herschel, we detected the J = 1-0 transition of 36ArH+ as a moderately strong absorption line initially associated with an unidentified carrier.2 In both cases, the absorption feature is unique in its appearance at all velocity components associated with diffuse foreground molecular clouds, together with its conspicuous absence at velocities related to the denser sources themselves. Model calculations are able to reproduce the derived ArH+ column densities and suggest that argonium resides in the largely atomic, diffuse interstellar medium with a molecular fraction of no more than ˜10-3. The 38ArH+ isotopologue was also detected.Subsequent observations toward the continuum sources W51, W49, W31C, and G34.3+0.1 resulted in unequivocal detections of 36ArH+ absorption. Hence, argonium is a good probe of the transition zone between atomic and molecular gas, in particular in combination with OH+ and H2O+, whose abundances peak at a molecular fraction of ˜0.1. Moreover, argonium is a good indicator of an enhanced cosmic ray ionization rate. Therefore, it may be prominent toward, e.g., active galactic nuclei (AGNs) in addition to supernova remnants.
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| 8. |
- Schilke, P., et al.
(författare)
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Ubiquitous argonium (ArH$^{+}$) in the diffuse interstellar medium: A molecular tracer of almost purely atomic gas
- 2014
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 566:June, s. 1-12
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Tidskriftsartikel (refereegranskat)abstract
- Aims: We describe the assignment of a previously unidentified interstellar absorption line to ArH+ and discuss its relevance in the context of hydride absorption in diffuse gas with a low H2 fraction. The confidence of the assignment to ArH+ is discussed, and the column densities are determined toward several lines of sight. The results are then discussed in the framework of chemical models, with the aim of explaining the observed column densities. Methods: We fitted the spectral lines with multiple velocity components, and determined column densities from the line-to-continuum ratio. The column densities of ArH+ were compared to those of other species, tracing interstellar medium (ISM) components with different H2 abundances. We constructed chemical models that take UV radiation and cosmic ray ionization into account. Results: Thanks to the detection of two isotopologues, 36ArH+ and 38ArH+, we are confident about the carrier assignment to ArH+. NeH+ is not detected with a limit of [NeH+]/[ArH+] ≤ 0.1. The derived column densities agree well with the predictions of chemical models. ArH+ is a unique tracer of gas with a fractional H2 abundance of 10-4 - 10-3 and shows little correlation to H2O+, which traces gas with a fractional H2 abundance of ≈0.1. Conclusions: A careful analysis of variations in the ArH+, OH+, H2O+, and HF column densities promises to be a faithful tracer of the distribution of the H2 fractional abundance by providing unique information on a poorly known phase in the cycle of interstellar matter and on its transition from atomic diffuse gas to dense molecular gas traced by CO emission. Abundances of these species put strong observational constraints upon magnetohydrodynamical (MHD)simulations of the interstellar medium, and potentially could evolve into a tool characterizing the ISM. Paradoxically, the ArH+ molecule is a better tracer of almost purely atomic hydrogen gas than Hi itself, since Hi can also be present in gas with a significant molecular content, but ArH+ singles out gas that is >99.9% atomic.
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| 9. |
- Thomas, Richard, et al.
(författare)
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DISSOCIATIVE RECOMBINATION OF VIBRATIONALLY COLD CH+3 AND INTERSTELLAR IMPLICATIONS
- 2012
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Ingår i: Astrophysical Journal. - : The American Astronomical Society. - 0004-637X .- 1538-4357. ; 758:1, s. 55-
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Tidskriftsartikel (refereegranskat)abstract
- CH3+ is an important molecular ion in the astrochemistry of diffuse clouds, dense clouds, cometary comae, and planetary ionospheres. However, the rate of one of the major destruction mechanisms of CH3+, dissociative recombination (DR), has long been uncertain, hindering the use of CH3+ as an astrochemical probe. Here, we present the first absolute measurement of the DR of vibrationally cold CH3+, which has been made using the heavy storage ring CRYRING in Stockholm, Sweden. From our collision-energy-dependent cross sections, we infer a thermal rate constant of k(T) = 6.97(+/- 0.03) x 10(-7)(T/300)(-0.61(+/- 0.01)) cm(3) s(-1) over the region 10 K <= T <= 1000 K. At low collision energies, we have measured the branching fractions of the DR products to be CH3 (0.00(- 0.00)(+ 0.01)), CH2 + H (0.35(-0.01)(+ 0.01)), CH + 2H (0.20(-0.02)(+0.02)), CH + H-2 (0.10(-0.01)(+0.01)), and C + H-2 + H (0.35(-0.02)(+ 0.01)), indicating that two or more C-H bonds are broken in 65% of all collisions. We also present vibrational calculations which indicate that the CH3+ ions in the storage ring were relaxed to the vibrational ground state by spontaneous emission during the storage time. Finally, we discuss the implications of these new measurements for the observation of CH3+ in regions of the diffuse interstellar medium where CH+ is abundant.
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