198 research outputs found
Observations of T-Tauri Stars using HST-GHRS: I. Far Ultraviolet Emission Lines
We have analyzed GHRS data of eight CTTS and one WTTS. The GHRS data consists
of spectral ranges 40 A wide centered on 1345, 1400, 1497, 1550, and 1900 A.
These UV spectra show strong SiIV, and CIV emission, and large quantities of
sharp (~40 km/s) H2 lines. All the H2 lines belong to the Lyman band and all
the observed lines are single peaked and optically thin. The averages of all
the H2 lines centroids for each star are negative which may indicate that they
come from an outflow. We interpret the emission in H2 as being due to
fluorescence, mostly by Ly_alpha, and identify seven excitation routes within 4
A of that line. We obtain column densities (10^12 to 10^15 cm^-2) and optical
depths (~1 or less) for each exciting transition. We conclude that the
populations are far from being in thermal equilibrium. We do not observe any
lines excited from the far blue wing of Ly_alpha, which implies that the
molecular features are excited by an absorbed profile. SiIV and CIV (corrected
for H2 emission) have widths of ~200 km/s, and an array of centroids
(blueshifted lines, centered, redshifted). These characteristics are difficult
to understand in the context of current models of the accretion shock. For DR
Tau we observe transient strong blueshifted emission, perhaps the a result of
reconnection events in the magnetosphere. We also see evidence of multiple
emission regions for the hot lines. While CIV is optically thin in most stars
in our sample, SiIV is not. However, CIV is a good predictor of SiIV and H2
emission. We conclude that most of the flux in the hot lines may be due to
accretion processes, but the line profiles can have multiple and variable
components.Comment: 67 pages, 19 figures, Accepted in Ap
Herschel Survey of Galactic OH+, H2O+, and H3O+: Probing the Molecular Hydrogen Fraction and Cosmic-Ray Ionization Rate
In diffuse interstellar clouds the chemistry that leads to the formation of
the oxygen bearing ions OH+, H2O+, and H3O+ begins with the ionization of
atomic hydrogen by cosmic rays, and continues through subsequent hydrogen
abstraction reactions involving H2. Given these reaction pathways, the observed
abundances of these molecules are useful in constraining both the total
cosmic-ray ionization rate of atomic hydrogen (zeta_H) and molecular hydrogen
fraction, f(H2). We present observations targeting transitions of OH+, H2O+,
and H3O+ made with the Herschel Space Observatory along 20 Galactic sight lines
toward bright submillimeter continuum sources. Both OH+ and H2O+ are detected
in absorption in multiple velocity components along every sight line, but H3O+
is only detected along 7 sight lines. From the molecular abundances we compute
f(H2) in multiple distinct components along each line of sight, and find a
Gaussian distribution with mean and standard deviation 0.042+-0.018. This
confirms previous findings that OH+ and H2O+ primarily reside in gas with low
H2 fractions. We also infer zeta_H throughout our sample, and find a log-normal
distribution with mean log(zeta_H)=-15.75, (zeta_H=1.78x10^-16 s^-1), and
standard deviation 0.29 for gas within the Galactic disk, but outside of the
Galactic center. This is in good agreement with the mean and distribution of
cosmic-ray ionization rates previously inferred from H3+ observations.
Ionization rates in the Galactic center tend to be 10--100 times larger than
found in the Galactic disk, also in accord with prior studies.Comment: 76 pages, 25 figures, 6 tables; accepted for publication in Ap
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
The ALMA-PILS survey: complex nitriles towards IRAS 16293-2422
Context. Complex organic molecules are readily detected in the inner regions of the gaseous envelopes of forming protostars. Their detection is crucial to understanding the chemical evolution of the Universe and exploring the link between the early stages of star formation and the formation of solar system bodies, where complex organic molecules have been found in abundance. In particular, molecules that contain nitrogen are interesting due to the role nitrogen plays in the development of life and the compact scales such molecules have been found to trace around forming protostars. Aims. The goal of this work is to determine the inventory of one family of nitrogen-bearing organic molecules, complex nitriles (molecules with a -C N functional group) towards two hot corino sources in the low-mass protostellar binary IRAS 16293-2422. This work explores the abundance differences between the two sources, the isotopic ratios, and the spatial extent derived from molecules containing the nitrile functional group. Methods. Using data from the Protostellar Interferometric Line Survey (PILS) obtained with ALMA, we determine abundances and excitation temperatures for the detected nitriles. We also present a new method for determining the spatial structure of sources with high line density and large velocity gradients-Velocity-corrected INtegrated emission (VINE) maps. Results. We detect methyl cyanide (CH3CN) as well as five of its isotopologues, including CHD2CN, which is the first detection in the interstellar medium (ISM). We also detect ethyl cyanide (C2H5CN), vinyl cyanide (C2H3CN), and cyanoacetylene (HC3N). We find that abundances are similar between IRAS 16293A and IRAS 16293B on small scales except for vinyl cyanide which is only detected towards the latter source. This suggests an important difference between the sources either in their evolutionary stage or warm-up timescales. We also detect a spatially double-peaked emission for the first time in molecular emission in the A source, suggesting that this source is showing structure related to a rotating toroid of material. Conclusions. With high-resolution observations, we have been able to show for the first time a number of important similarities and differences in the nitrile chemistry in these objects. These illustrate the utility of nitriles as potential tracers of the physical conditions in star-forming regions
Envelope structure of deeply embedded young stellar objects in the Serpens Molecular Cloud
Aperture synthesis and single-dish (sub) millimeter molecular lines and
continuum observations reveal in great detail the envelope structure of deeply
embedded young stellar objects (SMM1, SMM2, SMM3, SMM4) in the densely
star-forming Serpens Molecular Cloud. Resolved millimeter continuum emission
constrains the density structure to a radial power law with index -2.0 +/- 0.5,
and envelope masses of 8.7, 3.0, and 5.3 M_sol for SMM1, SMM3, and SMM4. The
core SMM2 does not seem to have a central condensation and may not have formed
a star yet. The molecular line observations can be described by the same
envelope model, if an additional, small amount of warm (100 K) material is
included. This probably corresponds to the inner few hundred AU of the envelope
were the temperature is high. In the interferometer beam, the molecular lines
reveal the inner regions of the envelopes, as well as interaction of the
outflow with the surrounding envelope. Bright HCO+ and HCN emission outlines
the cavities, while SiO and SO trace the direct impact of the outflow on
ambient gas. Taken together, these observations provide a first comprehensive
view of the physical and chemical structure of the envelopes of deeply embedded
young stellar objects in a clustered environment on scales between 1000 and
10,000 AU.Comment: 46 pages, incl. 12 postscript figures, uses ApJ latex and psfig
macro
Nonthermal Emission from a Supernova Remnant in a Molecular Cloud
In evolved supernova remnants (SNRs) interacting with molecular clouds, such
as IC 443, W44, and 3C391, a highly inhomogeneous structure consisting of a
forward shock of moderate Mach number, a cooling layer, a dense radiative shell
and an interior region filled with hot tenuous plasma is expected. We present a
kinetic model of nonthermal electron injection, acceleration and propagation in
that environment and find that these SNRs are efficient electron accelerators
and sources of hard X- and gamma-ray emission. The energy spectrum of the
nonthermal electrons is shaped by the joint action of first and second order
Fermi acceleration in a turbulent plasma with substantial Coulomb losses.
Bremsstrahlung, synchrotron, and inverse Compton radiation of the nonthermal
electrons produce multiwavelength photon spectra in quantitative agreement with
the radio and the hard emission observed by ASCA and EGRET from IC 443. We
distinguish interclump shock wave emission from molecular clump shock wave
emission accounting for a complex structure of molecular cloud. Spatially
resolved X- and gamma- ray spectra from the supernova remnants IC 443, W44, and
3C391 as might be observed with BeppoSAX, Chandra XRO, XMM, INTEGRAL and GLAST
would distinguish the contribution of the energetic lepton component to the
gamma-rays observed by EGRET.Comment: 14 pages, 4 figure, Astrophysical Journal, v.538, 2000 (in press
Complex organic molecules in low-mass protostars on Solar System scales -- II. Nitrogen-bearing species
The chemical inventory of planets is determined by the physical and chemical
processes that govern the early phases of star formation. The aim is to
investigate N-bearing complex organic molecules towards two Class 0 protostars
(B1-c and S68N) at millimetre wavelengths with ALMA. Next, the results of the
detected N-bearing species are compared with those of O-bearing species for the
same and other sources. ALMA observations in Band 6 ( 1 mm) and Band 5
( 2 mm) are studied at 0.5" resolution, complemented by Band 3
( 3 mm) data in a 2.5" beam. NH2CHO, C2H5CN, HNCO, HN13CO, DNCO,
CH3CN, CH2DCN, and CHD2CN are identified towards the investigated sources.
Their abundances relative to CH3OH and HNCO are similar for the two sources,
with column densities that are typically an order of magnitude lower than those
of O-bearing species. The largest variations, of an order of magnitude, are
seen for NH2CHO abundance ratios with respect to HNCO and CH3OH and do not
correlate with the protostellar luminosity. In addition, within uncertainties,
the N-bearing species have similar excitation temperatures to those of
O-bearing species ( 100 300 K). The similarity of most abundances
with respect to HNCO, including those of CH2DCN and CHD2CN, hints at a shared
chemical history, especially the high D/H ratio in cold regions prior to star
formation. However, some of the variations in abundances may reflect the
sensitivity of the chemistry to local conditions such as temperature (e.g.
NH2CHO), while others may arise from differences in the emitting areas of the
molecules linked to their different binding energies in the ice. The two
sources discussed here add to the small number of sources with such a detailed
chemical analysis on Solar System scales. Future JWST data will allow a direct
comparison between the ice and gas abundances of N-bearing species.Comment: Accepted to A&A, 41 pages, 37 figure
The cometary composition of a protoplanetary disk as revealed by complex cyanides
Observations of comets and asteroids show that the Solar Nebula that spawned
our planetary system was rich in water and organic molecules. Bombardment
brought these organics to the young Earth's surface, seeding its early
chemistry. Unlike asteroids, comets preserve a nearly pristine record of the
Solar Nebula composition. The presence of cyanides in comets, including 0.01%
of methyl cyanide (CH3CN) with respect to water, is of special interest because
of the importance of C-N bonds for abiotic amino acid synthesis. Comet-like
compositions of simple and complex volatiles are found in protostars, and can
be readily explained by a combination of gas-phase chemistry to form e.g. HCN
and an active ice-phase chemistry on grain surfaces that advances
complexity[3]. Simple volatiles, including water and HCN, have been detected
previously in Solar Nebula analogues - protoplanetary disks around young stars
- indicating that they survive disk formation or are reformed in situ. It has
been hitherto unclear whether the same holds for more complex organic molecules
outside of the Solar Nebula, since recent observations show a dramatic change
in the chemistry at the boundary between nascent envelopes and young disks due
to accretion shocks[8]. Here we report the detection of CH3CN (and HCN and
HC3N) in the protoplanetary disk around the young star MWC 480. We find
abundance ratios of these N-bearing organics in the gas-phase similar to
comets, which suggests an even higher relative abundance of complex cyanides in
the disk ice. This implies that complex organics accompany simpler volatiles in
protoplanetary disks, and that the rich organic chemistry of the Solar Nebula
was not unique.Comment: Definitive version of the manuscript is published in Nature, 520,
7546, 198, 2015. This is the author's versio
Cosmic-ray propagation in molecular clouds
Cosmic-rays constitute the main ionising and heating agent in dense,
starless, molecular cloud cores. We reexamine the physical quantities necessary
to determine the cosmic-ray ionisation rate (especially the cosmic ray spectrum
at E < 1 GeV and the ionisation cross sections), and calculate the ionisation
rate as a function of the column density of molecular hydrogen. Available data
support the existence of a low-energy component (below about 100 MeV) of
cosmic-ray electrons or protons responsible for the ionisation of diffuse and
dense clouds. We also compute the attenuation of the cosmic-ray flux rate in a
cloud core taking into account magnetic focusing and magnetic mirroring,
following the propagation of cosmic rays along flux tubes enclosing different
amount of mass and mass-to-flux ratios. We find that mirroring always dominates
over focusing, implying a reduction of the cosmic-ray ionisation rate by a
factor of 3-4 depending on the position inside the core and the magnetisation
of the core.Comment: To appear in "Cosmic Rays in Star-Forming Environments", Proceedings
of the 2nd Session of the Sant Cugat Forum on Astrophysics. D. F. Torres and
O. Reimer (Editors), 2013, Springer, 25 pages, 11 figure
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