585 research outputs found
CH 3 GHz Observations of the Galactic Center
A 3 3 map of the Galactic Center was made at 9\arcmin resolution
and 10\arcmin spacing in the CH , J=1/2, F=1-1 transition at
3335 MHz. The CH emission shows a velocity extent that is nearly that of the
CO(1-0) line, but the CH line profiles differ markedly from the CO. The 3335
MHz CH transition primarily traces low-density molecular gas and our
observations indicate that the mass of this component within 30 pc of
the Galactic Center is 9 10 M. The CO-H
conversion factor obtained for the low-density gas in the mapped region is
greater than that thought to apply to the dense molecular gas at the Galactic
Center. In addition to tracing the low-density molecular gas at the Galactic
Center, the CH spectra show evidence of emission from molecular clouds along
the line of sight both in the foreground and background. The scale height of
these clouds ranges from 27 - 109 pc, consistent with previous work based on
observations of molecular clouds in the inner Galaxy.Comment: 29 pages, 12 figure
Multi--Pressure Polytropes as Models for the Structure and Stability of Molecular Clouds. I. Theory
Molecular clouds are supported by thermal pressure, magnetic pressure, and
turbulent pressure. Each of these can be modeled with a polytropic equation of
state, so that overall the total pressure is the sum of the individual
components. We model the turbulent pressure as being due to a superposition of
Alfven waves. The theory of polytropes is generalized to allow for the flow of
entropy in response to a perturbation, as expected for the entropy associated
with wave pressure. The equation of state of molecular clouds is "soft", so
that the properties of the clouds are generally governed by the conditions at
the surface. In general, the polytropes are not isentropic, and this permits
large density and pressure drops to occur between the center and the edge of
the polytropes, as is observed.Comment: Submitted to ApJ with 10 figure
Potential Variations in the Interstellar N I Abundance
We present Far Ultraviolet Spectroscopic Explorer (FUSE) and Space Telescope
Imaging Spectrograph observations of the weak interstellar N I doublet at 1160
Angstroms toward 17 high-density sight lines [N(Htot)>=10^21 cm^-2]. When
combined with published data, our results reveal variations in the fractional N
I abundance showing a systematic deficiency at large N(Htot). At the FUSE
resolution (~20 km s^-1), the effects of unresolved saturation cannot be
conclusively ruled out, although O I at 1356 Angstroms shows little evidence of
saturation. We investigated the possibility that the N I variability is due to
the formation of N_2 in our mostly dense regions. The 0-0 band of the c'_4
^1Sigma^+_u - X ^1Sigma^+_g transition of N_2 at 958 Angstroms should be easily
detected in our FUSE data; for 10 of the denser sight lines, N_2 is not
observed at a sensitivity level of a few times 10^14 cm^-2. The observed N I
variations are suggestive of an incomplete understanding of nitrogen chemistry.
Based on observations made with the NASA-CNES-CSA Far Ultraviolet
Spectroscopic Explorer, which is operated for NASA by the Johns Hopkins
University under NASA contract NAS 5-32985, and the NASA/ESA Hubble Space
Telescope, obtained from the Multimission Archive at the Space Telescope
Science Institute, which is operated by the Association of Universities for
Research in Astronomy, Inc., under the NASA contract NAS 5-26555.Comment: 12 pages, 3 figures, accepted for publication in ApJ Letter
Magneto-Acoustic Waves of Small Amplitude in Optically Thin Quasi-Isentropic Plasmas
The evolution of quasi-isentropic magnetohydrodynamic waves of small but
finite amplitude in an optically thin plasma is analyzed. The plasma is assumed
to be initially homogeneous, in thermal equilibrium and with a straight and
homogeneous magnetic field frozen in. Depending on the particular form of the
heating/cooling function, the plasma may act as a dissipative or active medium
for magnetoacoustic waves, while Alfven waves are not directly affected. An
evolutionary equation for fast and slow magnetoacoustic waves in the single
wave limit, has been derived and solved, allowing us to analyse the wave
modification by competition of weakly nonlinear and quasi-isentropic effects.
It was shown that the sign of the quasi-isentropic term determines the scenario
of the evolution, either dissipative or active. In the dissipative case, when
the plasma is first order isentropically stable the magnetoacoustic waves are
damped and the time for shock wave formation is delayed. However, in the active
case when the plasma is isentropically overstable, the wave amplitude grows,
the strength of the shock increases and the breaking time decreases. The
magnitude of the above effects depends upon the angle between the wave vector
and the magnetic field. For hot (T > 10^4 K) atomic plasmas with solar
abundances either in the interstellar medium or in the solar atmosphere, as
well as for the cold (T < 10^3 K) ISM molecular gas, the range of temperature
where the plasma is isentropically unstable and the corresponding time and
length-scale for wave breaking have been found.Comment: 14 pages, 10 figures. To appear in ApJ January 200
GYES, a multifibre spectrograph for the CFHT
We have chosen the name of GYES, one of the mythological giants with one
hundred arms, offspring of Gaia and Uranus, for our instrument study of a
multifibre spectrograph for the prime focus of the Canada-France-Hawaii
Telescope. Such an instrument could provide an excellent ground-based
complement for the Gaia mission and a northern complement to the HERMES project
on the AAT. The CFHT is well known for providing a stable prime focus
environment, with a large field of view, which has hosted several imaging
instruments, but has never hosted a multifibre spectrograph. Building upon the
experience gained at GEPI with FLAMES-Giraffe and X-Shooter, we are
investigating the feasibility of a high multiplex spectrograph (about 500
fibres) over a field of view 1 degree in diameter. We are investigating an
instrument with resolution in the range 15000 to 30000, which should provide
accurate chemical abundances for stars down to 16th magnitude and radial
velocities, accurate to 1 km/s for fainter stars. The study is led by
GEPI-Observatoire de Paris with a contribution from Oxford for the study of the
positioner. The financing for the study comes from INSU CSAA and Observatoire
de Paris. The conceptual study will be delivered to CFHT for review by October
1st 2010.Comment: Contributed talk at the Gaia ELSA conference 2010, S\`evres 7-11 June
2010, to be published on the EAS Series, Editors: C. Turon, F. Arenou & F.
Meynadie
Spectroscopic survey of the Galaxy with Gaia I. Design and performance of the Radial Velocity Spectrometer
The definition and optimisation studies for the Gaia satellite spectrograph,
the Radial Velocity Spectrometer (RVS), converged in late 2002 with the
adoption of the instrument baseline. This paper reviews the characteristics of
the selected configuration and presents its expected performance. The RVS is a
2.0 by 1.6 degree integral field spectrograph, dispersing the light of all
sources entering its field of view with a resolving power R=11 500 over the
wavelength range [848, 874] nm. The RVS will continuously and repeatedly scan
the sky during the 5 years of the Gaia mission. On average, each source will be
observed 102 times over this period. The RVS will collect the spectra of about
100-150 million stars up to magnitude V~17-18. At the end of the mission, the
RVS will provide radial velocities with precisions of ~2 km/s at V=15 and
\~15-20 km/s at V=17, for a solar metallicity G5 dwarf. The RVS will also
provide rotational velocities, with precisions (at the end of the mission) for
late type stars of sigma_vsini ~5 km/s at V~15 as well as atmospheric
parameters up to V~14-15. The individual abundances of elements such as Silicon
and Magnesium, vital for the understanding of Galactic evolution, will be
obtained up to V~12-13. Finally, the presence of the 862.0 nm Diffuse
Interstellar Band (DIB) in the RVS wavelength range will make it possible to
derive the three dimensional structure of the interstellar reddening.Comment: 17 pages, 9 figures, accepted for publication in MNRAS. Fig. 1,2,4,5,
6 in degraded resolution; available in full resolution at
http://blackwell-synergy.com/links/doi/10.1111/j.1365-2966.2004.08282.x/pd
Very Cold Gas and Dark Matter
We have recently proposed a new candidate for baryonic dark matter: very cold
molecular gas, in near-isothermal equilibrium with the cosmic background
radiation at 2.73 K. The cold gas, of quasi-primordial abundances, is condensed
in a fractal structure, resembling the hierarchical structure of the detected
interstellar medium.
We present some perspectives of detecting this very cold gas, either directly
or indirectly. The H molecule has an "ultrafine" structure, due to the
interaction between the rotation-induced magnetic moment and the nuclear spins.
But the lines fall in the km domain, and are very weak. The best opportunity
might be the UV absorption of H in front of quasars. The unexpected cold
dust component, revealed by the COBE/FIRAS submillimetric results, could also
be due to this very cold H gas, through collision-induced radiation, or
solid H grains or snowflakes. The -ray distribution, much more
radially extended than the supernovae at the origin of cosmic rays
acceleration, also points towards and extended gas distribution.Comment: 16 pages, Latex pages, crckapb macro, 3 postscript figures, uuencoded
compressed tar file. To be published in the proceeedings of the
"Dust-Morphology" conference, Johannesburg, 22-26 January, 1996, D. Block
(ed.), (Kluwer Dordrecht
Impact of the COVID-19 Pandemic on a Cancer Fast-Track Programme
Introduction: The COVID-19 pandemic has disrupted many aspects of clinical practice in oncology, particularly regarding early cancer diagnosis, sparking public health concerns that possible delays could increase the proportion of patients diagnosed at advanced stages. In 2009, a cancer fast-track program (CFP) was implemented at the Clinico-Malvarrosa Health Department in Valencia, Spain with the aim of shortening waiting times between suspected cancer symptoms, diagnosis and therapy initiation. Objectives: The study aimed to explore the effects of the COVID-19 pandemic on our cancer diagnosis fast-track program. Methods: The program workflow (patients included and time periods) was analysed from the beginning of the state of alarm on March 16th, 2020 until March 15th, 2021. Data was compared with data from the same period of time from the year before (2019). Results: During the pandemic year, 975 suspected cancer cases were submitted to the CFP. The number of submissions only decreased during times of highest COVID-19 incidence and stricter lockdown, and overall, referrals were slightly higher than in the previous 2 years. Cancer diagnosis was confirmed in 197 (24.1%) cases, among which 33% were urological, 23% breast, 16% gastrointestinal and 9% lung cancer. The median time from referral to specialist appointment was 13 days and diagnosis was reached at a median of 18 days. In confirmed cancer cases, treatment was started at around 30 days from time of diagnosis. In total, 61% of cancer disease was detected at early stage, 20% at locally advanced stage, and 19% at advanced stage, displaying time frames and case proportions similar to pre-pandemic years. Conclusions: Our program has been able to maintain normal flow and efficacy despite the challenges of the current pandemic, and has proven a reliable tool to help primary care physicians referring suspected cancer patients.S
Molecular Hydrogen in the Ionized Region of Planetary Nebulae
This paper presents an analysis of the concentration of the hydrogen molecule
inside the ionized region of planetary nebulae. The equations corresponding to
the ionization and chemical equilibria of H, H+, H-, H2, H2+, and H3+ are
coupled with the equations of ionization and thermal balance for a photoionized
atomic gas. Forty different reactions related to the formation or the
destruction of these species are included. The presence of dust is taken into
account, since grains act as catalysts for the production of H2, as well as
shield the molecules against the stellar ionizing radiation. We analyze the
effect of the stellar ionizing continuum, as well as of the gas and grain
properties on the calculated H2 mass. It is shown that a significant
concentration of H2 can survive inside the ionized region of planetary nebulae,
mostly in the inner region of the recombination zone. The total H2 to total
hydrogen mass ratio inside the ionized region increases with the central star
temperature, and, depending on the PN physical conditions, it can be of the
order of 10^-6 or even higher. The increase of the recombination zone with the
stellar temperature can account for such correlation. This can explain why the
H2 emission is more frequently observed in bipolar planetary nebulae (Gatley's
rule), since this kind of object has typically hotter stars. Applying our
results for the planetary nebula NGC 6720, we obtain an H2 to hydrogen mass
ratio similar to the value obtained from the observed H2 line emission.Comment: 13 pages, 4 figures. Accepted for publication in Ap
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