Molecular inventories and chemical evolution of low-mass protostellar envelopes

By J. Jørgensen, F. Schoeier, E. Dishoeck, L. Observatory, S. Observatory. 2003

199 citations recorded by the source corpus.

This paper presents the first substantial study of the chemistry of the envelopes around a sample of 18 low-mass pre- and protostellar objects for which physical properties have previously been derived from radiative transfer modeling of their dust continuum emission. Single-dish line observations of 24 transitions of 9 molecular species (not counting isotopes) including HCO + , N 2 H + , CS, SO, SO 2 , HCN, HNC, HC 3 N and CN are reported. The line intensities are used to constrain the molecular abundances by comparison to Monte Carlo radiative transfer modeling of the line strengths. In general the nitrogen-bearing species together with HCO + and CO cannot be fitted by a constant fractional abundance when the lowest excitation transitions are included, but require radial dependences of their chemistry since the intensity of the lowest excitation lines are systematically underestimated in such models. A scenario is suggested in which these species are depleted in a specific region of the envelope where the density is high enough that the freeze-out timescale is shorter than the dynamical timescale and the temperature low enough that the molecule is not evaporated from the icy grain mantles. This can be simulated by a “drop” abundance profile with standard (undepleted) abundances in the inner- and outermost regions and a drop in abundance in between where the molecule freezes out. An empirical chemical network is constructed on the basis of correlations between the abundances of various species. For example, it is seen that the HCO + and CO abundances are linearly correlated, both increasing with decreasing envelope mass. This is shown to be the case if the main formation route of HCO + is through reactions between CO and H$_3^+$, and if the CO abundance still is low enough that reactions between H$_3^+$ and N 2 are the main mechanism responsible for the removal of H$_3^+$. Species such as CS, SO and HCN show no trend with envelope mass. In particular no trend is seen between “evolutionary stage” of the objects and the abundances of the main sulfur- or nitrogen-containing species. Among the nitrogen-bearing species abundances of CN, HNC and HC 3 N are found to be closely correlated, which can be understood from considerations of the chemical network. The CS/SO abundance ratio is found to correlate with the abundances of CN and HC 3 N, which may reflect a dependence on the atomic carbon abundance. An anti-correlation is found between the deuteration of HCO + and HCN, reflecting different temperature dependences for gas-phase deuteration mechanisms. The abundances are compared to other protostellar environments. In particular it is found that the abundances in the cold outer envelope of the previously studied class 0 protostar IRAS 16293-2422 are in good agreement with the average abundances for the presented sample of class 0 objects.

Subfields: Astronomy and Astrophysics, Atmospheric Science

View Paper: https://www.semanticscholar.org/paper/93550e8553b954df7ce9165db6c66782bdb36e04

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In Astronomy and Astrophysics

1. E. Dishoeck
2. P. Caselli
3. E. Bergin
4. M. Tafalla
5. N. Evans II
6. Simon Bruderer
7. P. Myers
8. K. Oberg

In Atmospheric Science

1. P. Caselli
2. E. Dishoeck
3. E. Bergin
4. M. Tafalla
5. K. Oberg
6. Simon Bruderer
7. P. Myers
8. S. Doty

Citations in the corpus

• Water in star-forming regions with Herschel (WISH). II. Evolution of 557 GHz 110-101 emission in low-mass protostars (2012, background): “Furthermore, these objects have been the targets of numerous physical and chemical surveys (e.g., Hogerheijde et al. 1999; Shirley et al. 2000; Jørgensen et al. 2002, 2004b, 2007; Emprechtinger et al. 2009; Kristensen et al. 2010a,b).”
• [Si II], [Fe II], [C II], and H2 Emission from Massive Star-forming Regions (2006, background): “At larger depths, we leave the PDR and enter the very opaquemolecular cores where the chemistry and heating are dominated by cosmic rays and where heavy gas-phase elements like O andCmay freeze out on grain surfaces to form ices (e.g., Bergin et al. 2000; Jørgensen et al. 2004).”
• Competition between CO and N2 Desorption from Interstellar Ices (2005, methodology): “The detailed chemical network of N2H formation and destruction is described in detail by Jørgensen et al. (2004), showing that the chemistries of CO, N2, N2H , HCO , and are inti- H3 1 Raymond and Beverly Sackler Laboratory for Astrophysics at Leiden Observatory, Postbus 9513, 2300 RA, Leiden,…”
• A Keplerian disk around a Class 0 source: ALMA observations of VLA1623A (2013, methodology): “From JCMT observations toward VLA1623, it is difficult to determine the position of absorbing material given the dense and active region (Jørgensen et al. 2004).”
• PROSAC: a submillimeter array survey of low-mass protostars - II. The mass evolution of envelopes, disks, and stars from the class 0 through I stages (2009, background): “…seen significant progress with the advent of new data and more detailed dust radiative transfer models (e.g., Hogerheijde & Sandell, 2000; Looney et al., 2003; Jørgensen et al., 2004a, 2005a; Jørgensen, 2004; Schöier et al., 2004; Eisner et al., 2005; Brinch et al., 2007; Lommen et al., 2008).”
• CHEMISTRY IN INFRARED DARK CLOUD CLUMPS: A MOLECULAR LINE SURVEY AT 3 mm (2011, background): “According to ion-molecule schemes of dense molecular clouds (e.g., Snyder et al. 1977; Jørgensen et al. 2004), N2H + and HCO+ are formed primary through reactions with H+3 : H+3 +N2 → N2H + +H2 (14) H+3 +CO → HCO + +H2 .”
• Evolution of Chemistry and Molecular Line Profiles during Protostellar Collapse (2004, background): “Jørgensen et al. (2004b) used a simple “drop” function, which reasonably approximates this profile, to simulate C18O lines to fit actual observations.”
• Cold Dark Clouds: The Initial Conditions for Star Formation (2007, background): “…key observable ions (N2H + and HCO+) will also decrease in abundance as the parent molecules freeze-out; however, CO is a major destroyer of N2H + and as CO disappears from the gas there is a subsequent increase in abundance of N2H + (Aikawa et al. 2001; Bergin et al. 2002; Jørgensen et al. 2004).”
• PROSAC: A Submillimeter Array Survey of Low-Mass Protostars. I. Overview of Program: Envelopes, Disks, Outflows, and Hot Cores (2007, methodology): “…of 100 AU) can be expressed as h 870i ¼ 0:5 cos MD 0:1 M RD 100 AU 2 : ð2Þ For example, a lower limit to the disk mass of 0.3 M was derived for NGC 1333 IRAS 2A from 3 mm observations by Jørgensen et al. (2004b), whereas the disk size was estimated to be 150AU (radius) in Jørgensen et al. (2005a).”