Excitation and abundance study of CO+ in the interstellar medium

Observations of CO+ suggest column densities on the order 10^12 cm^-2 that can not be reproduced by many chemical models. CO+ is more likely to be destroyed than excited in collisions with hydrogen. An anomalous excitation mechanism may thus have to be considered when interpreting CO^+ observations. Chemical models are used to perform a parameter study of CO^+ abundances. Line fluxes are calculated for N(CO+)=10^12 cm^-2 and different gas densities and temperatures using a non-LTE escape probability method. The chemical formation and destruction rates are considered explicitly in the detailed balance equations of the radiative transfer. In addition, the rotational levels of CO+ are assumed to be excited upon chemical formation according to a formation temperature. It is found, that chemical models are generally able to produce high fractional CO+ abundances (x(CO+) =10^-10). In a far-ultraviolet (FUV) dominated environment, however, high abundances of CO+ are only produced in regions with a Habing field G0 > 100 and T(kin) > 600 K, posing a strong constraint on the gas temperature. For gas densities >10^6 cm^-3 and temperatures > 600 K, the combination of chemical and radiative transfer analysis shows little effect on intensities of CO+ lines with upper levels N_up <= 3. Significantly different line fluxes are calculated with an anomalous excitation mechanism, however, for transitions with higher upper levels and densities >10^6 cm ^ -3. The Herschel Space Observatory is able to reveal such effects in the terahertz wavelength regime. Ideal objects to observe are protoplanetary disks with densities 10^6 cm^-3. It is finally suggested that the CO+ chemistry may be well understood and that the abundances observed so far can be explained with a high enough gas temperature and a proper geometry.Comment: 9 pages, 7 figure

Rotational excitation of methylidynium (CH+) by a helium atom at high temperature

We aim to obtain accurate rate coefficients for the collisional excitation of CH+ by He for high gas temperatures. The ab initio coupled-cluster [CCSD(T)] approximation was used to compute the interaction potential energy. Cross sections are then derived in the close coupling (CC) approach and rate coefficients inferred by averaging these cross sections over a Maxwell-Boltzmann distribution of kinetic energies. Cross sections are calculated up to 10'000 cm^-1 for J ranging from 0 to 10. Rate coefficients are obtained at high temperatures up to 2000 K.Comment: 4 pages, 3 figures, table with rate coefficients, accepted for publication by A&

Energetic radiation and the sulfur chemistry of protostellar envelopes: Submillimeter interferometry of AFGL 2591

CONTEXT: The chemistry in the inner few thousand AU of accreting envelopes around young stellar objects is predicted to vary greatly with far-UV and X-ray irradiation by the central star. Aim We search for molecular tracers of high-energy irradiation by the protostar in the hot inner envelope. METHODS: The Submillimeter Array (SMA) has observed the high-mass star forming region AFGL 2591 in lines of CS, SO, HCN, HCN(v2=1), and HC15N with 0.6" resolution at 350 GHz probing radial scales of 600-3500 AU for an assumed distance of 1 kpc. The SMA observations are compared with the predictions of a chemical model fitted to previous single-dish observations. RESULTS: The CS and SO main peaks are extended in space at the FWHM level, as predicted in the model assuming protostellar X-rays. However, the main peak sizes are found smaller than modeled by nearly a factor of 2. On the other hand, the lines of CS, HCN, and HC15N, but not SO and HCN(v2=1), show pedestal emissions at radii of about 3500 AU that are not predicted. All lines except SO show a secondary peak within the approaching outflow cone. A dip or null in the visibilities caused by a sharp decrease in abundance with increasing radius is not observed in CS and only tentatively in SO. CONCLUSIONS: The emission of protostellar X-rays is supported by the good fit of the modeled SO and CS amplitude visibilities including an extended main peak in CS. The broad pedestals can be interpreted by far-UV irradiation in a spherically non-symmetric geometry, possibly comprising outflow walls on scales of 3500 -- 7000 AU. The extended CS and SO main peaks suggest sulfur evaporation near the 100 K temperature radius.Comment: Astronomy and Astrophysics, in pres

Water destruction by X-rays in young stellar objects

We study the H2O chemistry in star-forming environments under the influence of a central X-ray source and a central far ultraviolet (FUV) radiation field. The gas-phase water chemistry is modeled as a function of time, hydrogen density and X-ray flux. To cover a wide range of physical environments, densities between n_H = 10^4-10^9 cm^-3 and temperatures between T = 10-1000 K are studied. Three different regimes are found: For T < 100 K, the water abundance is of order 10^-7-10^-6 and can be somewhat enhanced or reduced due to X-rays, depending on time and density. For 100 K < T < 250 K, H2O is reduced from initial x(H2O) ~ 10^-4 following ice evaporation to x(H2O) ~ 10^-6 for F_X > 10^-3 ergs s-1 cm^-2 (t = 10^4 yrs) and for F_X > 10^-4 ergs s^-1 cm^-2 (t = 10^5 yrs). At higher temperatures (T > 250 K) and hydrogen densities, water can persist with x(H2O) ~ 10^-4 even for high X-ray fluxes. The X-ray and FUV models are applied to envelopes around low-mass Class 0 and I young stellar objects (YSOs). Water is destroyed in both Class 0 and I envelopes on relatively short timescales (t ~ 5000 yrs) for realistic X-ray fluxes, although the effect is less prominent in Class 0 envelopes due to the higher X-ray absorbing densities there. FUV photons from the central source are not effective in destroying water. The average water abundance in Class I sources for L_X > 10^27 ergs s^-1 is predicted to be x(H2O) < 10^-6.Comment: 12 pages, 14 figures, Accepted for publication in A&

Photodesorption of water ice: a molecular dynamics study

Absorption of ultraviolet radiation by water ice coating interstellar grains can lead to dissociation and desorption of the ice molecules. These processes are thought to be important in the gas-grain chemistry in molecular clouds and protoplanetary disks, but very few quantitative studies exist. We compute the photodesorption efficiencies of amorphous water ice and elucidate the mechanisms by which desorption occurs. Classical molecular dynamics calculations were performed for a compact amorphous ice surface at 10 K thought to be representative of interstellar ice. Dissociation and desorption of H2O molecules in the top six monolayers are considered following absorption into the first excited electronic state with photons in the 1300-1500 Angstrom range. The trajectories of the H and OH photofragments are followed until they escape or become trapped in the ice. The probability for H2O desorption per absorbed UV photon is 0.5-1% in the top three monolayers, then decreases to 0.03% in the next two monolayers, and is negligible deeper into the ice. The main H2O removal mechanism in the top two monolayers is through separate desorption of H and OH fragments. Removal of H2O molecules from the ice, either as H2O itself or its products, has a total probability of 2-3% per absorbed UV photon in the top two monolayers. In the third monolayer the probability is about 1% and deeper into the ice the probability of photodesorption falling to insignificant numbers. The probability of any removal of H2O per incident photon is estimated to be 3.7x10^-4, with the probability for photodesorption of intact H2O molecules being 1.4x10^-4 per incident photon. When no desorption occurs, the H and OH products can travel up to 70 and 60 Angstroms inside or on top of the surface during which they can react with other species.Comment: 12 pages, 10 figures, A&A, in pres

Tracing high energy radiation with molecular lines near deeply embedded protostars

Submillimeter lines of CN, NO, CO+ and SO+, and upper limits on SH+ and N2O are observed with the James Clerk Maxwell Telescope in two high-mass and up to nine low-mass young stellar objects and compared with chemical models. Constant fractional abundances derived from radiative transfer modeling of the line strengths are x(CN) ~ a few x 10^{-11}-10^{-8}, x(NO) ~ 10^{-9}-10^{-8} and x(CO+) ~ 10^{-12}-10^{-10}. SO+ has abundances of a few x 10^{-11} in the high-mass objects and upper limits of ~ 10^{-12}-10^{-11} in the low-mass sources. All abundances are up to 1-2 orders of magnitude higher if the molecular emission is assumed to originate mainly from the inner region (< 1000 AU) of the envelope. For high-mass sources, the CN, SO+ and CO+ abundances and abundance ratios are best explained by an enhanced far-ultraviolet (FUV) field impacting gas at temperatures of a few hundred K. The observed column densities require that this region of enhanced FUV has scales comparable to the observing beam, such as in a geometry in which the enhanced FUV irradiates outflow walls. For low-mass sources, the required temperatures within the FUV models of T > 300 K are much higher than found in models, so that an X-ray enhanced region close to the protostar (r < 500 AU) is more plausible. The observed abundances imply X-ray fluxes for the Class 0 objects of L_X ~ 10^{29}-10^{31} erg s^{-1}, comparable to those observed from low-mass Class I protostars. Spatially resolved data are needed to clearly distinguish the effects of FUV and X-rays for individual species.Comment: accepted by A&A (02/01/2007

Tracing FUV Radiation in the Embedded Phase of Star Formation

Molecules containing one or a few hydrogen atoms and a heavier atom (hydrides) have been predicted to trace FUV radiation. In some chemical models, FUV emission by the central object or protostar of a star forming region greatly enhances some of the hydride abundances. Two massive regions, W3 IRS5 and AFGL 2591, have been observed in hydride lines by HIFI onboard the {\it Herschel Space Observatory}. We use published results as well as new observations of CH$^+$ towards W3 IRS5. Molecular column densities are derived from ground state absorption lines, radiative transfer modeling or rotational diagrams. Models assuming no internal FUV are compared with two-dimensional models including FUV irradiation of outflow walls. We confirm that the effect of FUV is clearly noticeable and greatly improves the fit. The most sensitive molecules to FUV irradiation are CH$^+$ and OH$^+$, enhanced in abundance by many orders of magnitude. Modeling in addition also full line radiative transfer, Bruderer et al (2010b) achieve good agreement of a two-dimensional FUV model with observations of CH$^+$ in AFGL 2591. It is concluded that CH$^+$ and OH$^+$ are good FUV tracers in star-forming regions.Comment: EAS Publications Series, 5th Zermatt conference on Conditions and impact of star formation: New results with Herschel and beyond (invited), submitte