48 research outputs found
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&
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
Infrared Molecular Starburst Fingerprints in Deeply Obscured (Ultra)Luminous Infrared Galaxy Nuclei
High-resolution spectra of the Spitzer Space Telescope show vibration-rotation absorption bands of gaseous C_2H_2, HCN, and CO_2 molecules toward a sample of deeply obscured (U)LIRG nuclei. The observed bands reveal the presence of dense (n ≳ 10^7 cm^(-3)), warm (T_(ex) = 200-700 K) molecular gas with high column densities of these molecules ranging from a few 10^(15) to 10^(17) cm^(-2). Abundances relative to H_2, inferred from the silicate optical depth, range from ~10^(-7) to 10^(-6) and show no correlation with temperature. Theoretical studies show that the high abundances of both C_2H_2 and HCN exclude an X-ray dominated region (XDR) associated with the toroid surrounding an AGN as the origin of this dense warm molecular gas. Galactic massive protostars in the so-called hot-core phase have similar physical characteristics with comparable high abundances of C_2H_2, HCN, and CO_2 in the hot phase. However, the abundances of C_2H_2 and HCN and the C_2H_2/CO_2 and HCN/CO_2 ratios are much higher toward the (U)LIRGs in the cooler (T_(ex) ≾ 400 K) phase. We suggest that the warm dense molecular gas revealed by the mid-IR absorption lines is associated with a phase of deeply embedded star formation, where the extreme pressures and densities of the nuclear starburst environment have inhibited the expansion of H II regions and the global disruption of the star-forming molecular cloud cores and have "trapped" the star formation process in an "extended" hot-core phase
Influence of UV radiation from a massive YSO on the chemistry of its envelope
We have studied the influence of far ultraviolet (UV) radiation from a
massive young stellar object (YSO) on the chemistry of its own envelope by
extending the models of Doty et al. (2002) to include a central source of UV
radiation. The models are applied to the massive star-forming region AFGL 2591
for different inner UV field strengths. Depth-dependent abundance profiles for
several molecules are presented and discussed. We predict enhanced column
densities for more than 30 species, especially radicals and ions. Comparison
between observations and models is improved with a moderate UV field incident
on the inner envelope, corresponding to an enhancement factor G0~10-100 at 200
AU from the star with an optical depth tau~15-17. Subtle differences are found
compared with traditional models of Photon Dominated Regions (PDRs) because of
the higher temperatures and higher gas-phase H2O abundance caused by
evaporation of ices in the inner region. In particular, the CN/HCN ratio is not
a sensitive tracer of the inner UV field, in contrast with the situation for
normal PDRs: for low UV fields, the extra CN reacts with H2 in the inner dense
and warm region and produces more HCN. It is found that the CH+ abundance is
strongly enhanced and grows steadily with increasing UV field. High-J lines of
molecules like CN and HCN are most sensitive to the inner dense region where UV
radiation plays a role. Thus, even though the total column density affected by
UV photons is small, comparison of high-J and low-J lines can selectively trace
and distinguish the inner UV field from the outer one. In addition, future
Herschel-HIFI observations of hydrides can sensitively probe the inner UV
field.Comment: Accepted for publication in A&A. 13 pages, 10 figure