400 research outputs found
Hydrogen Isocyanide in Comet 73P/Schwassmann-Wachmann (Fragment B)
We present a sensitive 3-sigma upper limit of 1.1% for the HNC/HCN abundance
ratio in comet 73P/Schwassmann-Wachmann (Fragment B), obtained on May 10-11,
2006 using Caltech Submillimeter Observatory (CSO). This limit is a factor of
~7 lower than the values measured previously in moderately active comets at 1
AU from the Sun. Comet 73P/Schwassmann-Wachmann was depleted in most volatile
species, except of HCN. The low HNC/HCN ratio thus argues against HNC
production from polymers produced from HCN. However, thermal degradation of
macromolecules, or polymers, produced from ammonia and carbon compounds, such
as acetylene, methane, or ethane appears a plausible explanation for the
observed variations of the HNC/HCN ratio in moderately active comets, including
the very low ratio in comet 73P/Schwassmann-Wachmann reported here. Similar
polymers have been invoked previously to explain anomalous 14N/15N ratios
measured in cometary CN.Comment: 6 pages, 5 figures, 2 table
O and Ne K absorption edge structures and interstellar abundance towards Cyg X-2
We have studied the O and Ne absorption features in the X-ray spectrum of Cyg
X-2 observed with the Chandra LETG. The O absorption edge is represented by the
sum of three absorption-edge components within the limit of the energy
resolution and the photon counting statistics. Two of them are due to the
atomic O; their energies correspond to two distinct spin states of
photo-ionized O atoms. The remaining edge component is considered to represent
compound forms of oxide dust grains. Since Cyg X-2 is about 1.4 kpc above the
galactic disk, the H column densities can be determined by radio (21 cm and CO
emission line) and H alpha observations with relatively small uncertainties.
Thus the O abundance relative to H can be determined from the absorption edges.
We found that the dust scattering can affect the apparent depth of the edge of
the compound forms. We determined the amplitude of the effect, which we
consider is the largest possible correction factor. The ratio of column
densities of O in atomic to compound forms and the O total abundance were
respectively determined to be in the range 1.7^{+3.0}_{-0.9} to
2.8^{+5.1}_{-1.5} (ratio), and 0.63 +/- 0.12 solar to 0.74 +/- 0.14 solar
(total), taking into account the uncertainties in the dust-scattering
correction and in the ionized H column density. We also determined the Ne
abundance from the absorption edge to be 0.75 +/- 0.20 solar. These abundance
values are smaller than the widely-used solar values but consistent with the
latest estimates of solar abundance.Comment: 20 pages, 3 figures, AASTeX format. Accepted for publication in Ap
A Cold Nearby Cloud Inside the Local Bubble
The high-latitude Galactic H I cloud toward the extragalactic radio source 3C
225 is characterized by very narrow 21 cm emission and absorption indicative of
a very low H I spin temperature of about 20 K. Through high-resolution optical
spectroscopy, we report the detection of strong, very narrow Na I absorption
corresponding to this cloud toward a number of nearby stars. Assuming that the
turbulent H I and Na I motions are similar, we derive a cloud temperature of 20
(+6, -8) K (in complete agreement with the 21 cm results) and a line-of-sight
turbulent velocity of 0.37+/-0.08 km/s from a comparison of the H I and Na I
absorption linewidths. We also place a firm upper limit of 45 pc on the
distance of the cloud, which situates it well inside the Local Bubble in this
direction and makes it the nearest-known cold diffuse cloud discovered to date.Comment: 11 pages, 3 figures, accepted for publication in ApJ Letter
Millimetre continuum observations of comet C/2009 P1 (Garradd)
Little is known about the physical properties of the nuclei of Oort cloud
comets. Measuring the thermal emission of a nucleus is one of the few means for
deriving its size and constraining some of its thermal properties. We attempted
to measure the nucleus size of the Oort cloud comet C/2009 P1 (Garradd). We
used the Plateau de Bure Interferometer to measure the millimetric thermal
emission of this comet at 157 GHz (1.9 mm) and 266 GHz (1.1 mm). Whereas the
observations at 266 GHz were not usable due to bad atmospheric conditions, we
derived a 3-sigma upper limit on the comet continuum emission of 0.41 mJy at
157 GHz. Using a thermal model for a spherical nucleus with standard thermal
parameters, we found an upper limit of 5.6 km for the radius. The dust
contribution to our signal is estimated to be negligible. Given the water
production rates measured for this comet and our upper limit, we estimated that
Garradd was very active, with an active fraction of its nucleus larger than
50%.Comment: Accepted for publication in Astronomy & Astrophysics. 5 pages, 2
figure
A survey of volatile species in Oort cloud comets C/2001 Q4 (NEAT) and C/2002 T7 (LINEAR) at millimeter wavelengths
The line emission in the coma was measured in the comets C/2001 Q4 (NEAT) and
C/2002 T7 (LINEAR), that were observed on five consecutive nights, 7-11 May
2004, at heliocentric distances of 1.0 and 0.7 AU, respectively, by means of
high-resolution spectroscopy using the 10-m Submillimeter Telescope (SMT). We
present a search for six parent- and product-volatile species (HCN, H2CO, CO,
CS, CH3OH, and HNC) in both comets. Multiline observations of the CH3OH J = 5-4
series allow us to estimate the rotational temperature using the rotation
diagram technique. We derive rotational temperatures of 54(9) K for C/2001 Q4
(NEAT) and 119(34) K for C/2002 T7 (LINEAR) that are roughly consistent with
observations of other comets at similar distances from the Sun. The gas
production rates of material are computed using a spherically symmetric
molecular excitation code that includes collisions between neutrals and
electrons. We find an HCN production rate of 2.96(5)e26 molec.s-1 for comet
C/2001 Q4 (NEAT), corresponding to a mixing ratio with respect to H2O of
1.12(2)e-3. The mean HCN production rate during the observing period is
4.54(10)e26 molec.s-1 for comet C/2002 T7 (LINEAR), which gives a Q_HCN/Q_H2O
mixing ratio of 1.51(3)e-3. With systematically lower mixing ratios in comet
C/2001 Q4 (NEAT), production rate ratios of the observed species with respect
to H2O lie within the typical ranges of dynamically new comets in both objects.
We find a relative low abundance of CO in C/2001 Q4 (NEAT) compared to the
observed range in other comets based on millimeter/submillimeter observations,
and a significant upper limit on the CO production in C/2002 T7 (LINEAR) is
derived. Depletion of CO suggests partial evaporation from the surface layers
during previous visits to the outer Solar System and agrees with previous
measurements of dynamically new comets.Comment: 20 pages, 18 figures. Minor changes to match the published versio
Submillimetric spectroscopic observations of volatiles in comet C/2004 Q2 (Machholz)
We aim to determine the production rates of several parent and product
volatiles and the 12C/13C isotopic carbon ratio in the long-period comet C/2004
Q2 (Machholz), which is likely to originate from the Oort Cloud. The line
emission from several molecules in the coma was measured with high
signal-to-noise ratio in January 2005 at heliocentric distance of 1.2 AU by
means of high-resolution spectroscopic observations using the Submillimeter
Telescope (SMT). We have obtained production rates of several volatiles (CH3OH,
HCN, H13CN, HNC, H2CO, CO and CS) by comparing the observed and simulated
line-integrated intensities. Furthermore, multiline observations of the CH3OH
(7-6) series allow us to estimate the rotational temperature using the rotation
diagram technique. We find that the CH3OH population distribution of the levels
sampled by these lines can be described by a rotational temperature of 40 \pm 3
K. Derived mixing ratios relative to hydrogen cyanide are
CO/CH3OH/H2CO/CS/HNC/H13CN/HCN = 30.9/24.6/4.8/0.57/0.031/0.013/1 assuming a
pointing offset of 8" due to the uncertain ephemeris at the time of the
observations and the telescope pointing error. The measured relative molecular
abundances in C/2004 Q2 (Machholz) are between low- to typical values of those
obtained in Oort Cloud comets, suggesting that it has visited the inner solar
system previously and undergone thermal processing. The HNC/HCN abundance ratio
of ~3.1% is comparable to that found in other comets, accounting for the
dependence on the heliocentric distance, and could possibly be explained by
ion-molecule chemical processes in the low-temperature atmosphere. From a
tentative H13CN detection, the measured value of 97 \pm 30 for the H12CN/H13CN
isotopologue pair is consistent with a telluric value.Comment: 14 pages with 11 figures, abridged abstrac
Velocity and density spectra of the Small Magellanic Cloud
This paper reports results on the statistical analysis of HI turbulence in
the Small Magellanic Cloud (SMC). We use 21 cm channel maps, obtained with the
Australia Telescope Compact Array and the Parkes telescope, and analyze the
spectrum of observed intensity fluctuations as a function of the velocity slice
thickness. We confirm predictions by Lazarian & Pogosyan (2000) on the change
of the power law index and establish the spectra of 3-D density and velocity.
The obtained spectral indices, -3.3 and -3.4, are slightly more shallow than
the predictions for the Kolmogorov spectrum. This contrasts to the predictions
for the shock-type spectra that are steeper than the Kolmogorov one. The nature
of the energy injection in the SMC is unclear as no distinct energy injection
scales are observed up to the entire scale of the SMC.Comment: 5 pages, 1 figure, accepted for publication in the ApJ Letter
Measuring Molecular, Neutral Atomic, and Warm Ionized Galactic Gas Through X-Ray Absorption
We study the column densities of neutral atomic, molecular, and warm ionized
Galactic gas through their continuous absorption of extragalactic X-ray spectra
at |b| > 25 degrees. For N(H,21cm) < 5x10^20 cm^-2 there is an extremely tight
relationship between N(H,21cm) and the X-ray absorption column, N(xray), with a
mean ratio along 26 lines of sight of N(xray)/N(H,21cm) = 0.972 +- 0.022. This
is significantly less than the anticpated ratio of 1.23, which would occur if
He were half He I and half He II in the warm ionized component. We suggest that
the ionized component out of the plane is highly ionized, with He being mainly
He II and He III. In the limiting case that H is entirely HI, we place an upper
limit on the He abundance in the ISM of He/H <= 0.103.
At column densities N(xray) > 5x10^20 cm^-2, which occurs at our lower
latitudes, the X-ray absorption column N(xray) is nearly double N(H,21cm). This
excess column cannot be due to the warm ionized component, even if He were
entirely He I, so it must be due to a molecular component. This result implies
that for lines of sight out of the plane with |b| ~ 30 degrees, molecular gas
is common and with a column density comprable to N(H,21cm).
This work bears upon the far infrared background, since a warm ionized
component, anticorrelated with N(H,21cm), might produce such a background. Not
only is such an anticorrelation absent, but if the dust is destroyed in the
warm ionized gas, the far infrared background may be slightly larger than that
deduced by Puget et al. (1996).Comment: 1 AASTeX file, 14 PostScript figure files which are linked within the
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