23 research outputs found
Chemical fractionation of deuterium in the protosolar nebula
Understanding gas-grain chemistry of deuterium in star-forming objects may
help to explain their history and present state. We aim to clarify how
processes in ices affect the deuterium fractionation. In this regard, we
investigate a Solar-mass protostellar envelope using an astrochemical
rate-equation model that considers bulk-ice chem- istry. The results show a
general agreement with the molecular D/H abundance ratios observed in low-mass
protostars. The simultaneous processes of ice accumulation and rapid synthesis
of HD on grain surfaces in the prestellar core hampers the deuteration of icy
species. The observed very high D/H ratios exceeding 10 per cent, i.e., super-
deuteration, are reproduced for formaldehyde and dimethyl ether, but not for
other species in the protostellar envelope phase. Chemical transformations in
bulk ice lower D/H ratios of icy species and do not help explaining the
super-deuteration. In the protostellar phase, the D2O/HDO abundance ratio was
calculated to be higher than the HDO/H2O ratio owing to gas-phase chemistry.
Species that undergo evaporation from ices have high molecular D/H ratio and a
high gas-phase abundance.Comment: 11 pages, 4 tables, 6 figures; +3 figures in appendix. Accepted for
publication in MNRA
Gaussian Spectral Line Profiles of Astrophysical Masers
Calculations are performed to demonstrate the deviations from Gaussian that
occur in the spectral line profiles of a linear maser as a result of the
amplification process. Near-Gaussian profiles are presented for bright,
interstellar 22 GHz water masers obtained from high resolution Very Long
Baseline Array (VLBA) observations of W3 IRS 5. For the profiles to be so close
to Gaussian, the calculations indicate that these masers must originate in
quite hot gas with temperatures greater than 1200 K -- a conclusion that is
supportive of C-type shocks as the origin of these masers. In addition, the
degree of saturation of these masers must be less than approximately one-third,
from which it follows that the beaming angles are less than about 10^{-4} ster
and the actual luminosities are modest. If spectral profiles that are as close
to Gaussian as the profiles presented in this initial investigation are found
to occur widely, they can be valuable diagnostics for the environments of
astrophysical masers.Comment: 4 pages with 3 figures embedded in paper; uses AASTEX with
emulateapj5.sty; accepted for publication in ApJ (Letters
Milliarcsecond structure and variability of methanol maser emission in three high-mass protostars
{The variability study of 6.7\,GHz methanol masers has become a useful way to
improve our understanding of the physical conditions in high-mass star-forming
regions.} {Based on the single-dish monitoring using the Irbene telescopes, we
selected three sources with close sky positions.} {We imaged them using the
European Very Long Baseline Interferometer Network and searched available data
on VLBI archives to follow detailed changes in their structures and single
maser spot variability.} {All three targets show a few groups of maser
cloudlets of a typical size of 3.5\,mas and the majority of them show linear or
arched structures with velocity gradients of order 0.22\kms\,mas. The
cloudlets and overall source morphologies are remarkably stable on time scales
of 7-15\,yr supporting a scenario of variability due to changes in the maser
pumping rate.}Comment: 20 page
New ex-OH maser detections in the northern celestial hemisphere
Aims.Molecular masers, including methanol and hydroxyl masers, and in
particular the ones in excited rotational states (ex-OHmasers), are one of the
most informative tools for studying star-forming regions. So, the discovery, of
new maser sources in theseregions is of great importance. Many studies and
surveys of ex-OH maser sources have been carried out in the southern
celestialhemisphere, but only a few have been done in the northern hemisphere.
The specific aim of this work is to close this gap.Methods.The star-forming
regions in the northern hemisphere with known active methanol masers were
observed to search for newex-OH maser sources with the 32 m and 16 m radio
telescopes of the Ventspils International Radio Astronomy Centre
(VIRAC).Results.Three OH maser lines in the excited state at the 6035 MHz in
three northern hemisphere star-forming regions are detected.The maser
189.030+0.783 was previously known, but we suggest this maser is a possible
variable. We confirm recent detections ofthe ex-OH masers 85.41+0.00 and
90.92+1.49 by other authors. The magnetic field strength in the masering
regions is estimated byusing right circular polarization (RCP) and left
circular polarization (LCP) pair splitting. The high-velocity resolution
provides uswith an estimation of a comparatively small magnetic field strength
for the 189.030+0.783 and 90.92+1.49 star-forming regionsComment: 6 pages 1 figure 4 table
Subsurface chemistry of mantles of interstellar dust grains in dark molecular cores
Context. The abundances of many observed compounds in interstellar molecular
clouds still lack an explanation, despite extensive research that includes both
gas and solid (dust-grain surface) phase reactions. Aims. We aim to
qualitatively prove the idea that a hydrogen-poor subsurface chemistry on
interstellar grains is responsible for at least some of these chemical
"anomalies". This chemistry develops in the icy mantles when photodissociation
reactions in the mantle release free hydrogen, which escapes the mantle via
diffusion. This results in serious alterations of the chemical composition of
the mantle because pores in the mantle provide surfaces for reactions in the
new, hydrogen-poor environment. Methods. We present a simple kinetic model,
using existing astrochemical reaction databases. Gas phase, surface and
subsurface pore reactions are included, as are physical transformations of
molecules. Results. Our model produces significantly higher abundances for
various oxidized species than most other models. We also obtain quite good
results for some individual species that have adequate reaction network. Thus,
we consider that the hydrogen-poor mantle chemistry may indeed play a role in
the chemical evolution of molecular clouds. Conclusions. The significance of
outward hydrogen diffusion has to be proved by further research. A huge number
of solid phase reactions between many oxidized species is essential to obtain
good, quantitative modeling results for a comparison with observations. We
speculate that a variety of unobservable hydrogen-poor sulfur oxoacid
derivatives may be responsible for the "disappearance" of sulfur in dark cloud
cores.Comment: 12 pages, 4 tables, 1 figur
H2O Masers and Supersonic Turbulence
We use unpublished and published VLBI results to investigate the geometry and
the statistical properties of the velocity field traced by H2O masers in five
galactic regions of star formation -- Sgr B2(M), W49N, W51(MAIN), W51N, and
W3(OH). In all sources the angular distribution of the H2O hot spots
demonstrates approximate self-similarity (fractality) over almost four orders
of magnitude in scale, with the calculated fractal dimension d between
(approximately) 0.2 and 1.0. In all sources, the lower order structure
functions for the line-of-sight component of the velocity field are
satisfactorily approximated by power laws, with the exponents near their
classic Kolmogorov values for the high-Reynolds-number incompressible
turbulence. These two facts, as well as the observed significant excess of
large deviations of the two-point velocity increments from their mean values,
strongly suggest that the H2O masers in regions of star formation trace
turbulence. We propose a new conceptual model of these masers in which maser
hot spots originate at the sites of ultimate dissipation of highly supersonic
turbulence produced in the ambient gas by the intensive gas outflow from a
newly-born star. Due to the high brightness and small angular sizes of masing
hot spots and the possibility of measuring their positions and velocities with
high precision, they become a unique probe of supersonic turbulence.Comment: 40 pages with 14 total figures (figures 9 and 10 are multi-part)
Accepted for publication ApJ, December 20, 2002, Vol. 581, n
Maser Emission, Spectral and Photometric Variability of V 645 Cyg and LkHα 234
Maser variability in young stellar objects can be related to episodic accretion outbursts. Among the young objects associated with maser sources, V 645 Cyg and LkH 234 are few of those that can be investigated in the optical range. We report their infrared and optical photometric variability, variation of optical spectral lines (H ), and check for their possible correlation with the maser activity.Переменность мазеров может быть связана с аккрецией вещества на молодой звездный объект. Среди молодых объектов, связанных с мазерными источниками, V 645 Cyg и LkH 234 немногие из тех, излучение которых проявляется в оптическом диапазоне. Исследована их инфракрасная (ИК) и оптическая фотометрическая переменность, переменность линий оптической части спектра (Н ), а также возможная корреляция с мазерной активностью.Работа А. П. Бисяриной по обработке и анализу спектральных данных и данных ИК наблюдений выполнена при финансовой поддержке РФФИ в рамках научного проекта №18-32-00314. Работа А. М. Соболева и А. П. Бисяриной по организации и анализу сопутствующих мазерных наблюдений, а также работа С.Ю. Горды по получению и обработке данных оптических наблюдений выполнена при поддержке гранта РНФ (№ 18-12-00193)
Recent updates on the Maser Monitoring Organisation
The Maser Monitoring Organisation (M2O) is a research community of telescope operators, astronomy researchers and maser theoreticians pursuing a joint goal of reaching a deeper understanding of maser emission and exploring its variety of uses as tracers of astrophysical events. These proceedings detail the origin, motivations and current status of the M2O, as was introduced at the 2021 EVN symposium