Charging Up Interstellar Chemistry

by Paul Gilster on July 24, 2007

Scientists studying the chemistry of interstellar space have identified around 130 neutral molecules along with perhaps a dozen positively charged molecules, but it was only late last year that the first negatively charged molecule — anion — was found, consisting of six carbon atoms and one hydrogen atom. It was a significant find because logic seemed to suggest that molecules would have a hard time retaining extra electrons, and thus a negative charge, in a star-rich environment.

Now we have a new anion, found using data from the Green Bank Telescope in West Virginia. The molecule is negatively-charged octatetraynyl, consisting of eight carbon atoms and one hydrogen atom, and it’s been located in the envelope of gas around an old, evolved star known as IRC +10 216, about 550 light-years from Earth. That makes three anions found in less than a year and in a range of environments.

Negatively charged interstellar ions

Image: Astronomers using the Robert C. Byrd Green Bank Telescope found the negatively-charged form of octatetraynyl (C8H-) in a cold interstellar cloud (middle left) and in the gaseous envelope surrounding an old, evolved star (middle right). This is the largest negatively-charged molecule yet found in space. The scientists believe it probably is formed in steps, illustrated here, proceeding downward.

1. A molecule of C2H attaches to a molecule of C6H2, producing a molecule of C8H2 and a hydrogen atom.
2. Radiation (squiggly line) breaks one hydrogen atom from the C8H2, leaving C8H and a hydrogen atom.
3. Finally, an electron attaches itself to the C8H molecule, freeing a burst of radiation (overall glow seen around the molecule) and leaving the negatively-charged ion C8H-.

Credit: Bill Saxton, NRAO/AUI/NSF.

NASA’s Jan Hollis (GSFC) has this to say about this significance of the findings:

“Until recently, many theoretical models of how chemical reactions evolve in interstellar space have largely neglected the presence of anions. This can no longer be the case, and this means that there are many more ways to build large organic molecules in cosmic environments than have been explored.”

This is interesting work. What you do to identify such molecules is to figure out what radio frequencies are characteristic of the charged molecule, a task accomplished in a laboratory to nail down the frequency range the astronomers need to look for. A team from the Harvard-Smithsonian Center for Astrophysics (CfA) used such methods (and data from the GBT) to find the same anion in a cloud of molecular gas called TMC-1, which is in the constellation Taurus. It’s likely, based on this work, that many more negatively charged molecules exist in space.

Thus we keep learning more about the different ways complex organic and other molecular types can form, some of which may be precursors to life. The interstellar medium continues to give us clues about how widespread living things may be. The relevant papers are Remijan et al., “Detection of C8H- and Comparison with C8H toward IRC +10 216,” Astrophysical Journal Letters 664 (July 20, 2007), pp. L47-L50 (abstract) and Brünken et al., “Detection of the Carbon Chain Negative Ion C8H- in TMC-1,” in the same issue, pp. L43-L46 (abstract).

Related: University of Arizona astronomers have been examining the supergiant VY Canis Majoris, about 5000 light years from Earth and 25 times as massive as the Sun. The star is huge, but losing mass so quickly that it will be gone in a scant million years. The team have already detected a score of molecules never before found in interstellar space, including table salt (NaCl); a compound called phosphorus nitride (PN), which contains two of the five most necessary ingredients for life; molecules of HNC, which is a variant form of the organic molecule, hydrogen cyanide; and an ion molecule form of carbon monoxide that comes with a proton attached (HCO+). More in this news release.

Do life’s origins go back to the chemistry found in exotic places like the space around VY Canis Majoris? The paper is Ziurys et al., “Chemical complexity in the winds of the oxygen-rich supergiant star VY Canis Majoris,” Nature 447 (28 June 2007), pp. 1094-1097 (abstract).

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ljk August 14, 2007 at 16:22

Hot Gas in Space Mimics Life

http://bcast1.imaginova.com/t?r=2&ctl=19F4D:4A48D

But scientists are hesitant to say they are alive.

andy August 14, 2007 at 18:42

ljk: those kind of systems approach the fuzzy grey area between life and non-life. Is fire alive? It converts energy and can reproduce after all. Is a mule alive? It cannot reproduce…

Maybe other dusty, plasma-rich environments could support such interesting systems. The outer layers of red giants/supergiants perhaps.

ljk December 7, 2007 at 10:46

Detection of circumstellar CH2CHCN, CH2CN, CH3CCH and H2CS

Authors: M. Agundez, J. P. Fonfria, J. Cernicharo, J. R., Pardo, M. Guelin

(Submitted on 6 Dec 2007)

Abstract: We report on the detection of vinyl cyanide (CH2CHCN), cyanomethyl radical (CH2CN), methylacetylene (CH3CCH) and thioformaldehyde (H2CS) in the C-rich star IRC +10216. These species, which are all known to exist in dark clouds, are detected for the first time in the circumstellar envelope around an AGB star. The four molecules have been detected trough pure rotational transitions in the course of a 3 mm line survey carried out with the IRAM 30-m telescope. The molecular column densities are derived by constructing rotational temperature diagrams. A detailed chemical model of the circumstellar envelope is used to analyze the formation of these molecular species. We have found column densities in the range 5 x 10^(12)- 2 x 10^(13) cm^(-2), which translates to abundances relative to H2 of several 10^(-9). The chemical model is reasonably successful in explaining the derived abundances through gas phase synthesis in the cold outer envelope. We also find that some of these molecules, CH2CHCN and CH2CN, are most probably excited trough infrared pumping to excited vibrational states. The detection of these species stresses the similarity between the molecular content of cold dark clouds and C-rich circumstellar envelopes. However, some differences in the chemistry are indicated by the fact that in IRC +10216 partially saturated carbon chains are present at a lower level than those which are highly unsaturated, while in TMC-1 both types of species have comparable abundances.

Comments: 9 pages, 5 figures; accepted for publication in A&A

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0712.1029v1 [astro-ph]

Submission history

From: Marcelino Agundez [view email]

[v1] Thu, 6 Dec 2007 20:07:27 GMT (458kb)

http://arxiv.org/abs/0712.1029

ljk January 9, 2008 at 0:49

The c2d Spitzer Spectroscopic Survey of Ices Around Low-Mass Young Stellar Objects: I. H2O and the 5-8 um Bands

Authors: A. Boogert, K. Pontoppidan (Caltech), C. Knez (U. Maryland), F. Lahuis (Groningen), J. Kessler-Silacci (U. Texas), E. van Dishoeck (Leiden), G. Blake (Caltech), J. Augereau (Grenoble), S. Bisschop, S. Bottinelli (Leiden), T. Brooke, J. Brown (Caltech), A. Crapsi (Leiden), N. Evans II (U. Texas), H. Fraser (Glasgow), V. Geers (Leiden), T. Huard, J. Jorgensen (SAO), K. Oberg (Leiden), L. Allen (SAO), P. Harvey (U. Texas), D. Koerner (NAU), L. Mundy (U. Maryland), D. Padgett, A. Sargent (Caltech), K. Stapelfeldt (JPL)

(Submitted on 8 Jan 2008)

Abstract: With the goal to study the physical and chemical evolution of ices in solar-mass systems, a spectral survey is conducted of a sample of 41 low luminosity YSOs using 3-38 um Spitzer and ground-based spectra. The long-known 6.0 and 6.85 um bands are detected toward all sources, with the Class 0-type YSOs showing the deepest bands ever observed. In almost all sources the 6.0 um band is deeper than expected from the bending mode of pure solid H2O. The depth and shape variations of the remaining 5-7 um absorption indicate that it consists of 5 independent components, which, by comparison to laboratory studies, must be from at least 8 different carriers. Simple species are responsible for much of the absorption in the 5-7 um region, at abundances of 1-30% for CH3OH, 3-8% for NH3, 1-5% for HCOOH, ~6% for H2CO, and ~0.3% for HCOO- with respect to solid H2O. The 6.85 um band likely consists of one or two carriers, of which one is less volatile than H2O because its abundance relative to H2O is enhanced at lower H2O/tau_9.7 ratios. It does not survive in the diffuse interstellar medium (ISM), however. The similarity of the 6.85 um bands for YSOs and background stars indicates that its carrier(s) must be formed early in the molecular cloud evolution. If an NH4+ salt is the carrier its abundance with respect to solid H2O is typically 7%, and low temperature acid-base chemistry or cosmic ray induced reactions must have been involved in its formation. Possible origins are discussed for the carrier of an enigmatic, very broad absorption between 5 and 8 um. Finally, all the phenomena observed for ices toward massive YSOs are also observed toward low mass YSOs, indicating that processing of the ices by internal ultraviolet radiation fields is a minor factor in the early chemical evolution of the ices. [abridged]

Comments: Accepted for publication in ApJ. 22 pages, 18 b&w figures

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0801.1167v1 [astro-ph]

Submission history

From: Adwin Boogert [view email]

[v1] Tue, 8 Jan 2008 06:57:12 GMT (783kb)

http://arxiv.org/abs/0801.1167

ljk January 9, 2008 at 0:51

The c2d Spitzer spectroscopy survey of ices around low-mass young stellar objects, III: CH4

Authors: Karin I. Oberg, A. C. Adwin Boogert, Klaus M. Pontoppidan, Geoffrey A. Blake, Neal J. Evans, Fred Lahuis, Ewine F. van Dishoeck

(Submitted on 8 Jan 2008)

Abstract: CH4 is proposed to be the starting point of a rich organic chemistry. Solid CH4 abundances have previously been determined mostly toward high mass star forming regions. Spitzer/IRS now provides a unique opportunity to probe solid CH4 toward low mass star forming regions as well. Infrared spectra from the Spitzer Space Telescope are presented to determine the solid CH4 abundance toward a large sample of low mass young stellar objects. 25 out of 52 ice sources in the $c2d$ (cores to disks) legacy have an absorption feature at 7.7 um, attributed to the bending mode of solid CH4. The solid CH4 / H2O abundances are 2-8%, except for three sources with abundances as high as 11-13%. These latter sources have relatively large uncertainties due to small total ice column densities. Toward sources with H2O column densities above 2E18 cm-2, the CH4 abundances (20 out of 25) are nearly constant at 4.7+/-1.6%. Correlation plots with solid H2O, CH3OH, CO2 and CO column densities and abundances relative to H2O reveal a closer relationship of solid CH4 with CO2 and H2O than with solid CO and CH3OH. The inferred solid CH4 abundances are consistent with models where CH4 is formed through sequential hydrogenation of C on grain surfaces. Finally the equal or higher abundances toward low mass young stellar objects compared with high mass objects and the correlation studies support this formation pathway as well, but not the two competing theories: formation from CH3OH and formation in gas phase with subsequent freeze-out.

Comments: 27 pages, 7 figures, accepted by ApJ

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0801.1223v1 [astro-ph]

Submission history

From: Karin \”Oberg [view email]

[v1] Tue, 8 Jan 2008 13:28:14 GMT (136kb)

http://arxiv.org/abs/0801.1223

ljk January 22, 2008 at 13:37

The search for complex molecules in the ISM: a complete 3 mm line survey of Sgr B2-N and -M

Authors: A. Belloche, C. Comito, C. Hieret, K. M. Menten, H. S. P. Mueller, P. Schilke

(Submitted on 21 Jan 2008)

Abstract: Famous for the extraordinary richness of its molecular content, the Sgr B2 molecular cloud complex is the prime target in the long-standing search for ever more complex species. We have completed a molecular line survey of the hot dense cores Sgr B2(N) and Sgr B2(M) in the 3 mm wavelength range with the IRAM 30 m telescope. We performed the analysis of this huge data set by modeling the whole spectrum at once in the LTE approximation.

Ongoing analyses yield an average line density of about 100 features/GHz above 3 sigma for Sgr B2(N), emitted and/or absorbed by a total of 51 molecular species. We find lines from 60 rare isotopologues and from 41 vibrationally excited states in addition to the main species, vibrational ground state lines. For Sgr B2(M), we find about 25 features/GHz above 3 sigma, from 41 molecular species plus 50 isotopologues and 20 vibrationally excited states. Thanks to the constant updates to the Cologne Database for Molecular Spectroscopy, we are working our way through the assignment of the unidentified features, currently 40% and 50% above 3 sigma for Sgr B2(N) and Sgr B2(M), respectively.

Comments: 4 pages, 2 figures, to appear in the proceedings of the conference “Molecules in Space & Laboratory” held in Paris, France, 14-18 May 2007

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0801.3214v1 [astro-ph]

Submission history

From: Arnaud Belloche [view email]

[v1] Mon, 21 Jan 2008 16:16:51 GMT (244kb)

http://arxiv.org/abs/0801.3214

ljk January 22, 2008 at 13:39

Detection of amino acetonitrile in Sgr B2(N)

Authors: A. Belloche, K. M. Menten, C. Comito, H. S. P. Mueller, P. Schilke, J. Ott, S. Thorwirth, C. Hieret

(Submitted on 21 Jan 2008)

Abstract: Amino acids are building blocks of proteins and therefore key ingredients for the origin of life. The simplest amino acid, glycine, has long been searched for in the interstellar medium but has not been unambiguously detected so far. Since the search for glycine has turned out to be extremely difficult, we aimed at detecting a chemically related species (possibly a direct precursor), amino acetonitrile. With the IRAM 30m telescope we carried out a complete line survey of the hot core regions Sgr B2(N) and (M) in the 3 mm range, plus partial surveys at 2 and 1.3 mm. We analyzed our 30m line survey in the LTE approximation and modeled the emission of all known molecules simultaneously. We identified spectral features at the frequencies predicted for amino acetonitrile lines having intensities compatible with a unique rotation temperature. We also used the VLA to look for cold, extended emission from amino acetonitrile.

We detected amino acetonitrile in Sgr B2(N) in our 30m telescope line survey and conducted confirmatory observations of selected lines with the IRAM PdB and the ATCA interferometers. The emission arises from a known hot core, the Large Molecule Heimat, and is compact with a source diameter of 2 arcsec (0.08 pc). We derived a column density of 2.8e16 cm-2, a temperature of 100 K, and a linewidth of 7 km s-1. Based on the simultaneously observed continuum emission, we calculated a density of 1.7e8 cm-3, a mass of 2340 Msun, and an amino acetonitrile fractional abundance of 2.2e-9. The high abundance and temperature may indicate that amino acetonitrile is formed by grain surface chemistry. We did not detect any hot, compact amino acetonitrile emission toward Sgr B2(M) or any cold, extended emission toward Sgr B2, with column-density upper limits of 6e15 and 3e12-14 cm-2, respectively. (abridged)

Comments: 30 pages, 8 figures, accepted for publication in Astronomy and Astrophysics (A&A)

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0801.3219v1 [astro-ph]

Submission history

From: Arnaud Belloche [view email]

[v1] Mon, 21 Jan 2008 16:20:52 GMT (2037kb)

http://arxiv.org/abs/0801.3219

ljk October 6, 2008 at 0:03

Molecules in the disk orbiting the twin young suns of V4046 Sgr

Authors: Joel H. Kastner (Rochester Institute of Technology and Laboratoire d’Astrophysique de Grenoble), B. Zuckerman (Dept. of Physics & Astronomy, UCLA and UCLA Center for Astrobiology), Pierre Hily-Blant (Laboratoire d’Astrophysique de Grenoble), Thierry Forveille (Laboratoire d’Astrophysique de Grenoble)

(Submitted on 2 Oct 2008)

Abstract: We report the results of a mm-wave molecular line survey of the nearby (D ~ 70 pc), 12 Myr-old system V4046 Sgr — a tight (9 R_sun separation), short-period (2.42 day) binary with nearly equal component masses of ~0.9 M_sun — conducted with the 30 m telescope of the Institut de Radio Astronomie Millimetrique (IRAM).

We detected rotational transitions of 12CO 13CO, HCN, CN, and HCO+. The double-peaked CO line profiles of V4046 Sgr are well fit by a model invoking a Keplerian disk with outer radius of ~250 AU that is viewed at an inclination i = 35 degrees.

We infer minimum disk gas and dust masses of ~13 and ~20 Earth masses from the V4046 Sgr CO line and submm continuum fluxes, respectively.

The actual disk gas mass could be much larger if the gas-phase CO is highly depleted and/or 13CO is very optically thick. The overall similarity of the circumbinary disk of V4046 Sgr to the disk orbiting the single, ~8 Myr-old star TW Hya — a star/disk system often regarded as representative of the early solar nebula — indicates that gas giant planets are likely commonplace among close binary star systems.

Given the relatively advanced age and proximity of V4046 Sgr, these results provide strong motivation for future high-resolution imaging designed to ascertain whether a planetary system now orbits its twin suns.

Comments: 5 pages, 3 figures; to appear in Astronomy & Astrophysics

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0810.0472v1 [astro-ph]

Submission history

From: Joel Kastner [view email]

[v1] Thu, 2 Oct 2008 16:16:58 GMT (52kb)

http://arxiv.org/abs/0810.0472

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