4,172 research outputs found
Herschel observations in the ultracompact HII region Mon R2: Water in dense photon-dominated regions (PDRs)
Context. Monoceros R2, at a distance of 830 pc, is the only ultracompact Hii region (UC H_(II)) where the photon-dominated region (PDR) between
the ionized gas and the molecular cloud can be resolved with Herschel. Therefore, it is an excellent laboratory to study the chemistry in extreme
PDRs (G_0 > 10^5 in units of Habing field, n > 10^6 cm^9−3)).
Aims. Our ultimate goal is to probe the physical and chemical conditions in the PDR around the UC H_(II) Mon R2.
Methods. HIFI observations of the abundant compounds ^(13)CO, C^(18)O, o-H_2^(18)O, HCO^+, CS, CH, and NH have been used to derive the physical
and chemical conditions in the PDR, in particular the water abundance. The modeling of the lines has been done with the Meudon PDR code and
the non-local radiative transfer model described by Cernicharo et al.
Results. The ^(13)CO, C^(18)O, o-H^(18)_2O, HCO^+ and CS observations are well described assuming that the emission is coming from a dense (n =
5 × 10^6 cm^(−3), N(H_2) > 10^(22) cm^(−2)) layer of molecular gas around the H_(II) region. Based on our o-H^(18)_2O observations, we estimate an o-H_2O
abundance of ≈2 × 10^(−8). This is the average ortho-water abundance in the PDR. Additional H^(18)_2O and/or water lines are required to derive the
water abundance profile. A lower density envelope (n ~ 10^5 cm^(−3), N(H_2) = 2−5 × 10^(22) cm^(−2)) is responsible for the absorption in the NH 1_1 → 0_2
line. The emission of the CH ground state triplet is coming from both regions with a complex and self-absorbed profile in the main component.
The radiative transfer modeling shows that the ^(13)CO and HCO^+ line profiles are consistent with an expansion of the molecular gas with a velocity
law, v_e = 0.5 × (r/R_(out))^(−1) km s^(−1), although the expansion velocity is poorly constrained by the observations presented here.
Conclusions. We determine an ortho-water abundance of ≈2 × 10^(−8) in Mon R2. Because shocks are unimportant in this region and our estimate is
based on H^(18)_2O observations that avoids opacity problems, this is probably the most accurate estimate of the water abundance in PDRs thus far
Ghosts of Milky Way's past: the globular cluster ESO 37-1 (E 3)
Context. In the Milky Way, most globular clusters are highly conspicuous
objects that were found centuries ago. However, a few dozen of them are faint,
sparsely populated systems that were identified largely during the second half
of the past century. One of the faintest is ESO 37-1 (E 3) and as such it
remains poorly studied, with no spectroscopic observations published so far,
although it was discovered in 1976.
Aims. We investigate the globular cluster E 3 in an attempt to better
constrain its fundamental parameters. Spectroscopy of stars in the field of E 3
is shown here for the first time.
Methods. Deep, precise VI CCD photometry of E 3 down to V=26 mag is presented
and analysed. Low-resolution, medium signal-to-noise ratio spectra of nine
candidate members are studied to derive radial velocity and metallicity. Proper
motions from the UCAC4 catalogue are used to explore the kinematics of the
bright members of E 3.
Results. Isochrone fitting indicates that E 3 is probably very old, with an
age of about 13 Gyr; its distance from the Sun is nearly 10 kpc. It is also
somewhat metal rich with [Fe/H]=-0.7. Regarding its kinematics, our tentative
estimate for the proper motions is (-7.0+/-0.8, 3.5+/-0.3) mas/yr (or a
tangential velocity of 382+/-79 km/s) and for the radial velocity is 45+/-5
km/s, in the solar rest frame.
Conclusions. E 3 is one of the most intriguing globular clusters in the
Galaxy. Having an old age and being metal rich is clearly a peculiar
combination, only seen in a handful of objects like the far more conspicuous
NGC 104 (47 Tucanae). In addition, its low luminosity and sparse population
make it a unique template for the study of the final evolutionary phases in the
life of a star cluster. Unfortunately, E 3 is among the most elusive and
challenging known globular clusters because field contamination severely
hampers spectroscopic studies.Comment: 7 pages, 6+1 figures, 2 tables. Accepted for publication in Astronomy
and Astrophysics. Minor change
Detection of CO+ in the nucleus of M82
We present the detection of the reactive ion CO+ towards the prototypical
starburst galaxy M82. This is the first secure detection of this short-lived
ion in an external galaxy. Values of [CO+]/[HCO+]>0.04 are measured across the
inner 650pc of the nuclear disk of M82. Such high values of the [CO+]/[HCO+]
ratio had only been previously measured towards the atomic peak in the
reflection nebula NGC7023. This detection corroborates that the molecular gas
reservoir in the M82 disk is heavily affected by the UV radiation from the
recently formed stars. Comparing the column densities measured in M82 with
those found in prototypical Galactic photon-dominated regions (PDRs), we need
\~20 clouds along the line of sight to explain our observations. We have
completed our model of the molecular gas chemistry in the M82 nucleus. Our PDR
chemical model successfully explains the [CO+]/[HCO+] ratios measured in the
M~82 nucleus but fails by one order of magnitude to explain the large measured
CO+ column densities (~1--4x10^{13} cm^{-2}). We explore possible routes to
reconcile the chemical model and the observations.Comment: 12 pages, 2 figure
Molecular line probes of activity in galaxies
The use of specific tracers of the dense molecular gas phase can help to
explore the feedback of activity on the interstellar medium (ISM) in galaxies.
This information is a key to any quantitative assessment of the efficiency of
the star formation process in galaxies. We present the results of a survey
devoted to probe the feedback of activity through the study of the excitation
and chemistry of the dense molecular gas in a sample of local universe
starbursts and active galactic nuclei (AGNs). Our sample includes also 17
luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs). From the
analysis of the LIRGs/ULIRGs subsample, published in Gracia-Carpio et al.(2007)
we find the first clear observational evidence that the star formation
efficiency of the dense gas, measured by the L_FIR/L_HCN ratio, is
significantly higher in LIRGs and ULIRGs than in normal galaxies. Mounting
evidence of overabundant HCN in active environments would even reinforce the
reported trend, pointing to a significant turn upward in the Kennicutt-Schmidt
law around L_FIR=10^11 L_sun. This result has major implications for the use of
HCN as a tracer of the dense gas in local and high-redshift luminous infrared
galaxies.Comment: 4 pages, 2 figures, contributed paper to Far-Infrared Workshop 07
(FIR 2007
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
Insights into the Carbon chemistry of Mon R2
Aiming to learn about the chemistry of the dense PDR around the ultracompact
(UC) HII region in Mon R2, we have observed a series of mm-wavelength
transitions of C3H2 and C2H. In addition, we have traced the distribution of
other molecules, such as H13CO+, SiO, HCO, and HC3N. These data, together with
the reactive ions recently detected, have been considered to determine the
physical conditions and to model the PDR chemistry. We then identified two kind
of molecules. The first group, formed by the reactive ions (CO+, HOC+) and
small hydrocarbons (C2H, C3H2), traces the surface layers of the PDR and is
presumably exposed to a high UV field (hence we called it as "high UV", or
HUV). HUV species is expected to dominate for visual absorptions 2 < Av < 5
mag. A second group (less exposed to the UV field, and hence called "low UV",
or LUV) includes HCO and SiO, and is mainly present at the edges of the PDR (Av
> 5 mag). While the abundances of the HUV molecules can be explained by gas
phase models, this is not the case for the studied LUV ones. Although some
efficient gas-phase reactions might be lacking, grain chemistry sounds like a
probable mechanism able to explain the observed enhancement of HCO and SiO.
Within this scenario, the interaction of UV photons with grains produces an
important effect on the molecular gas chemistry and constitutes the first
evidence of an ionization front created by the UC HII region carving its host
molecular cloud. The physical conditions and kinematics of the gas layer which
surrounds the UC HII region were derived from the HUV molecules. Molecular
hydrogen densities > 4 10^6 cm^(-3) are required to reproduce the observations.
Such high densities suggest that the HII region could be pressure-confined by
the surrounding high density molecular gas.Comment: 32 pages, 8 figures. Accepted by Astrophysical Journa
Complex organic molecules in strongly UV-irradiated gas
We investigate the presence of COMs in strongly UV-irradiated interstellar
molecular gas. We have carried out a complete millimetre line survey using the
IRAM30m telescope towards the edge of the Orion Bar photodissociation region
(PDR), close to the H2 dissociation front, a position irradiated by a very
intense far-UV (FUV) radiation field. These observations have been complemented
with 8.5 arcsec resolution maps of the H2CO 5(1,5)-4(1,4) and C18O 3-2 emission
at 0.9 mm. Despite being a harsh environment, we detect more than 250 lines
from COMs and related precursors: H2CO, CH3OH, HCO, H2CCO, CH3CHO, H2CS, HCOOH,
CH3CN, CH2NH, HNCO, H13-2CO, and HC3N (in decreasing order of abundance). For
each species, the large number of detected lines allowed us to accurately
constrain their rotational temperatures (Trot) and column densities (N). Owing
to subthermal excitation and intricate spectroscopy of some COMs (symmetric-
and asymmetric-top molecules such as CH3CN and H2CO, respectively), a correct
determination of N and Trot requires building rotational population diagrams of
their rotational ladders separately. We also provide accurate upper limit
abundances for chemically related molecules that might have been expected, but
are not conclusively detected at the edge of the PDR (HDCO, CH3O, CH3NC,
CH3CCH, CH3OCH3, HCOOCH3, CH3CH2OH, CH3CH2CN, and CH2CHCN). A non-LTE LVG
excitation analysis for molecules with known collisional rate coefficients,
suggests that some COMs arise from different PDR layers but we cannot resolve
them spatially. In particular, H2CO and CH3CN survive in the extended gas
directly exposed to the strong FUV flux (Tk = 150-250 K and Td > 60 K), whereas
CH3OH only arises from denser and cooler gas clumps in the more shielded PDR
interior (Tk = 40-50 K). We find a HCO/H2CO/CH3OH = 1/5/3 abundance ratio.
These ratios are different from those inferred in hot cores and shocks.Comment: 29 pages, 22 figures, 17 tables. Accepted for publication in A&A
(abstract abridged
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