182 research outputs found
The first results from the Herschel-HIFI mission
This paper contains a summary of the results from the first years of
observations with the HIFI instrument onboard ESA's Herschel space observatory.
The paper starts by outlining the goals and possibilities of far-infrared and
submillimeter astronomy, the limitations of the Earth's atmosphere, and the
scientific scope of the Herschel-HIFI mission. The presentation of science
results from the mission follows the life cycle of gas in galaxies as grouped
into five themes: Structure of the interstellar medium, First steps in
interstellar chemistry, Formation of stars and planets, Solar system results
and Evolved stellar envelopes. The HIFI observations paint a picture where the
interstellar medium in galaxies has a mixed, rather than a layered structure;
the same conclusion may hold for protoplanetary disks. In addition, the HIFI
data show that exchange of matter between comets and asteroids with planets and
moons plays a large role. The paper concludes with an outlook to future
instrumentation in the far-infrared and submillimeter wavelength ranges.Comment: Invited review paper to appear in Advances of Space Research; 32
pages, 9 figures. See http://www.sron.rug.nl/~vdtak/adspres.pdf for a 14-page
journal-style versio
Recent Astrochemical Results on Star-Forming Regions
This review discusses recent results on the astrochemistry of (mostly
high-mass) star-forming regions. After an introduction on the use of chemistry
in astrophysics and some basic concepts of astrochemistry, specific results are
presented. Highlighted areas are the use of chemistry in the search for massive
circumstellar disks, the interaction of molecular clouds with cosmic rays, and
the feedback effects of protostellar irradiation on the parent molecular cloud.
The review concludes with a discussion of future observational opportunities.Comment: Contribution to proceedings of conference "Massive Star Formation:
Observations confront Theory" (Heidelberg 2007); 8 DIN A4 pages. Version 2:
some references adde
Star & planet formation: Upcoming opportunities in the space-based infrared
While ALMA and JWST are revolutionizing our view of star and planet formation
with their unprecedented sensitivity and resolution at submillimeter and
near-IR wavelengths, many outstanding questions can only be answered with
observations in the thermal (mid- and far-) infrared domain. Many of these
questions require space-based observations, to achieve the necessary
sensitivity and/or wavelength coverage. In particular, how do interstellar
clouds develop filamentary structures and dense cores? What are the masses and
luminosities of objects at the earliest stages of star formation? What are the
gas masses of planet-forming disks, and how do these disks disperse during
planet formation? How is refractory and volatile material distributed within
the disks, and how does this evolve with time? This article reviews how
upcoming and planned balloon-borne and space-based telescopes for the mid- and
far-infrared will address these questions, and outlines which further missions
will be needed beyond 2030, when the ELTs will be in full operation.Comment: Contribution to the proceedings of the 7th Chile-Cologne-Bonn
symposium: Physics and Chemistry of Star Formation; 5 pages. Version 2
includes useful comments on the original version; version 3 includes further
comment
Limits on the cosmic-ray ionization rate toward massive young stars
Recent models of the envelopes of seven massive protostars are used to
analyze observations of H3+ infrared absorption and H13CO+ submillimeter
emission lines toward these stars, and to constrain the cosmic-ray ionization
rate zeta. The H13CO+ gives best-fit values of zeta=(2.6+/- 1.8) x 10^-17 s^-1,
in good agreement with diffuse cloud models and with recent Voyager/Pioneer
data but factors of up to 7 lower than found from the H3+ data. No relation of
zeta with luminosity or total column density is found, so that local (X-ray)
ionization and shielding against cosmic rays appear unimportant for these
sources. The difference between the H3+ and H13CO+ results and the correlation
of N(H3+) with heliocentric distance suggest that intervening clouds contribute
significantly to the H3+ absorptions in the more distant regions. The most
likely absorbers are low-density (<~10^4 cm^-3) clouds with most carbon in
neutral form or in CO.Comment: To be published in A&A 358 (Letters); 4 pages including 3 figure
Observations and models of the embedded phase of high-mass star formation
This paper is a review and an update on recent work on the physical and
chemical structure of the envelopes of newly born massive stars, at the stages
preceding ultracompact H II regions. It discusses methods and results to
determine total mass, temperature and density structure, ionization rate, and
depth-dependent chemical composition.Comment: 8 pages incl 4 figures, to appear in "Hot Star Workshop III: The
Earliest Phases of Massive Star Birth" (ed. P.A. Crowther) (ASP). Uses
newpasp.sty (included
The chemistry of high-mass star formation
This paper reviews the chemistry of star-forming regions, with an emphasis on
the formation of high-mass stars. We first outline the basic molecular
processes in dense clouds, their implementation in chemical models, and
techniques to measure molecular abundances. Then, recent observational,
theoretical and laboratory developments are reviewed on the subjects of hot
molecular cores, cosmic-ray ionization, depletion and deuteration, and oxygen
chemistry. The paper concludes with a summary of outstanding problems and
future opportunities.Comment: 10 A4 pages, 1 colour figure; invited review, to appear in "Massive
Star Birth - A Crossroads of Astrophysics" (CUP), eds. R. Cesaroni, E.
Churchwell, M. Felli, and C.M. Walmsle
The ionization rates of galactic nuclei and disks from Herschel/HIFI observations of water and its associated ions
(Abridged) We present Herschel/HIFI spectra of the H2O 1113 GHz and H2O+ 1115
GHz lines toward five nearby prototypical starburst/AGN systems, and OH+ 971
GHz spectra toward three of these. The beam size of 20" corresponds to
resolutions between 0.35 and 7 kpc. The observed line profiles range from pure
absorption (NGC 4945, M82) to P-Cygni indicating outflow (NGC 253, Arp 220) and
inverse P-Cygni indicating infall (Cen A). The similarity of the H2O, OH+, and
H2O+ profiles to each other and to HI indicates that diffuse and dense gas
phases are well mixed. We estimate column densities assuming negligible
excitation (for absorption features) and using a non-LTE model (for emission
features), adopting calculated collision data for H2O and OH+, and rough
estimates for H2O+. Column densities range from ~10^13 to ~10^15 cm^-2 for each
species, and are similar between absorption and emission components, indicating
that the nuclear region does not contribute much to the emission in these
ground-state lines. The N(H2O)/N(H2O+) ratios of 1.4-5.6 indicate an origin of
the lines in diffuse gas, and the N(OH+)/N(H2O+) ratios of 1.6-3.1 indicate a
low H2 fraction (~11%) in the gas.
Adopting recent Galactic values for the average gas density and the
ionization efficiency, we find ionization rates for our sample galaxies of
~3x10^-16 s^-1 which are similar to the value for the Galactic disk, but ~10x
below that of the Galactic Center and ~100x below estimates for AGN from
excited-state H3O+ lines. We conclude that the ground-state lines of water and
its associated ions probe primarily non-nuclear gas in the disks of these
centrally active galaxies. Our data thus provide evidence for a decrease in
ionization rate by a factor of ~10 from the nuclei to the disks of galaxies, as
found before for the Milky Way.Comment: Accepted for publication in Astronomy & Astrophysics; 8 pages, 8
figure
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