685 research outputs found
Photometric stability analysis of the Exoplanet Characterisation Observatory
Photometric stability is a key requirement for time-resolved spectroscopic
observations of transiting extrasolar planets. In the context of the Exoplanet
Characterisation Observatory (EChO) mission design, we here present and
investigate means of translating spacecraft pointing instabilities as well as
temperature fluctuation of its optical chain into an overall error budget of
the exoplanetary spectrum to be retrieved. Given the instrument specifications
as of date, we investigate the magnitudes of these photometric instabilities in
the context of simulated observations of the exoplanet HD189733b secondary
eclipse.Comment: submitted to MNRA
Herschel observations of the Sgr B2 cores: Hydrides, warm CO, and cold dust
Sagittarius B2 (Sgr B2) is one of the most massive and luminous star-forming
regions in the Galaxy and shows chemical and physical conditions similar to
those in distant extragalactic starbursts. We present large-scale far-IR/submm
photometric images and spectroscopic maps taken with the PACS and SPIRE
instruments onboard Herschel. The spectra towards the Sgr B2 star-forming
cores, B2(M) and B2(N), are characterized by strong CO line emission, emission
lines from high-density tracers (HCN, HCO+, and H2S), [N II] 205 um emission
from ionized gas, and absorption lines from hydride molecules (OH+, H2O+, H2O,
CH+, CH, SH+, HF, NH, NH2, and NH3). The rotational population diagrams of CO
suggest the presence of two gas temperature components: an extended warm
component, which is associated with the extended envelope, and a hotter
component, which is seen towards the B2(M) and B2(N) cores. As observed in
other Galactic Center clouds, the gas temperatures are significantly higher
than the dust temperatures inferred from photometric images. We determined
far-IR and total dust masses in the cores. Non-local thermodynamic equilibrium
models of the CO excitation were used to constrain the averaged gas density in
the cores. A uniform luminosity ratio is measured along the extended envelope,
suggesting that the same mechanism dominates the heating of the molecular gas
at large scales. The detection of high-density molecular tracers and of strong
[N II] 205 um line emission towards the cores suggests that their morphology
must be clumpy to allow UV radiation to escape from the inner HII regions.
Together with shocks, the strong UV radiation field is likely responsible for
the heating of the hot CO component. At larger scales, photodissociation
regions models can explain both the observed CO line ratios and the uniform
L(CO)/LFIR luminosity ratios
The ISO LWS high resolution spectral survey towards Sagittarius B2
A full spectral survey was carried out towards the Giant Molecular Cloud
complex, Sagittarius B2 (Sgr B2), using the ISO Long Wavelength Spectrometer
Fabry-Perot mode. This provided complete wavelength coverage in the range
47-196 um (6.38-1.53 THz) with a spectral resolution of 30-40 km/s. This is an
unique dataset covering wavelengths inaccessible from the ground. It is an
extremely important region of the spectrum as it contains both the peak of the
thermal emission from dust, and crucial spectral lines of key atomic (OI, CII,
OIII, NII and NIII) and molecular species (NH3, NH2, NH, H2O, OH, H3O+, CH,
CH2, C3, HF and H2D+). In total, 95 spectral lines have been identified and 11
features with absorption depth greater than 3 sigma remain unassigned. Most of
the molecular lines are seen in absorption against the strong continuum,
whereas the atomic and ionic lines appear in emission (except for absorption in
the OI 63 um and CII 158 um lines). Sgr B2 is located close to the Galactic
Centre and so many of the features also show a broad absorption profile due to
material located along the line of sight. A full description of the survey
dataset is given with an overview of each detected species and final line lists
for both assigned and unassigned features.Comment: Accepted for publication in MNRA
Herschel-SPIRE-Fourier Transform Spectroscopy of the nearby spiral galaxy IC342
We present observations of the nearby spiral galaxy IC342 with the Herschel
Spectral and Photometric Imaging Receiver (SPIRE) Fourier Transform
Spectrometer. The spectral range afforded by SPIRE, 196-671 microns, allows us
to access a number of 12CO lines from J=4--3 to J=13--12 with the highest J
transitions observed for the first time. In addition we present measurements of
13CO, [CI] and [NII]. We use a radiative transfer code coupled with Bayesian
likelihood analysis to model and constrain the temperature, density and column
density of the gas. We find two 12CO components, one at 35 K and one at 400 K
with CO column densities of 6.3x10^{17} cm^{-2} and 0.4x10^{17} cm^{-2} and CO
gas masses of 1.26x10^{7} Msolar and 0.15x10^{7} Msolar, for the cold and warm
components, respectively. The inclusion of the high-J 12CO line observations,
indicate the existence of a much warmer gas component (~400 K) confirming
earlier findings from H_{2} rotational line analysis from ISO and Spitzer. The
mass of the warm gas is 10% of the cold gas, but it likely dominates the CO
luminosity. In addition, we detect strong emission from [NII] 205microns and
the {3}P_{1}->{3}P_{0} and {3}P_{2} ->{3}P_{1} [CI] lines at 370 and 608
microns, respectively. The measured 12CO line ratios can be explained by
Photon-dominated region (PDR) models although additional heating by e.g. cosmic
rays cannot be excluded. The measured [CI] line ratio together with the derived
[C] column density of 2.1x10^{17} cm^{-2} and the fact that [CI] is weaker than
CO emission in IC342 suggests that [CI] likely arises in a thin layer on the
outside of the CO emitting molecular clouds consistent with PDRs playing an
important role.Comment: 9 pages, 8 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Society (MNRAS
An Executive Appraisal of Courses Which Best Prepare One for General Management
This ongoing study summarizes 1980-81 data from 1158 newly promoted executives in the United States who answered this question: "Assuming the study of business administration best prepares a young person for a career in general management, how important are the following courses as a part of that preparation?" Business Communication-oral and written- was the course selected as Very Important more often than any of thirteen courses.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/68655/2/10.1177_002194368201900102.pd
Discovery of far-infrared pure rotational transitions of CH+ in NGC 7027
We report the discovery of the rotational spectrum of CH+ in the Infrared Space Observatory Long Wavelength Spectrometer (LWS) spectrum of the planetary nebula NGC 7027. The identification relies on a 1996 reanalysis of the LWS spectrum by Liu et al. and on new LWS data. The strong line at 179.62 μm (coinciding with the 212-101 transition of water vapor) and the lines at 119.90 and 90.03 μm (reported as unidentified by Liu et al.), whose frequencies are in the harmonic relation 2 : 3:4, are shown to arise from the J = 2-1, 3-2, and 4-3 rotational transitions of CH+. This identification is strengthened by the new LWS spectra of NGC 7027, which clearly show the next two rotational lines of CH+ at 72.140 and 60.247 μm. This is the first time that the pure rotational spectrum of CH+ has been observed. This discovery opens the possibility of probing the densest and warmest zones of photodissociation regions. We derive a rotational temperature for the CH+ lines of 150 ± 20 K and a CH+/CO abundance ratio of 2-6 × 10-4
Calibration of <i>Herschel</i> SPIRE FTS observations at different spectral resolutions
The SPIRE Fourier Transform Spectrometer on-board the Herschel Space Observatory had two standard spectral resolution modes for science observations: high resolution (HR) and low resolution (LR), which could also be performed in sequence (H+LR). A comparison of the HR and LR resolution spectra taken in this sequential mode revealed a systematic discrepancy in the continuum level. Analysing the data at different stages during standard pipeline processing demonstrates that the telescope and instrument emission affect HR and H+LR observations in a systematically different way. The origin of this difference is found to lie in the variation of both the telescope and instrument response functions, while it is triggered by fast variation of the instrument temperatures. As it is not possible to trace the evolution of the response functions using housekeeping data from the instrument subsystems, the calibration cannot be corrected analytically. Therefore, an empirical correction for LR spectra has been developed, which removes the systematic noise introduced by the variation of the response functions
A new infrared band in the Interstellar and Circumstellar Clouds: C_4 or C_4H?
We report on the detection with the Infrared Space Observatory (ISO) of a
molecular band at 57.5 microns (174 cm^{-1}) in carbon-rich evolved stars and
in Sgr B2. Taking into account the chemistry of these objects the most
likelihood carrier is a carbon chain. We tentatively assign the band to the
nu_5 bending mode of C_4 for which a wavenumber of 170-172.4 cm^{-1} has been
derived in matrix experiments (Withey et al. 1991). An alternate carrier might
be C_4H, although the frequency of its lowest energy vibrational bending mode,
nu_7, is poorly known (130-226 cm^{-1}). If the carrier is C_4, the derived
maximum abundance is nearly similar to that found for C_3 in the interstellar
and circumstellar media by Cernicharo, Goicoechea & Caux (2000). Hence,
tetra-atomic carbon could be one of the most abundant carbon chain molecules in
these media.Comment: 11 pages, 1 figure, accepted in ApJ Letter
Oxygen isotopic ratios in galactic clouds along the line of sight towards Sagittarius B2
As an independent check on previous measurements of the isotopic abundance of
oxygen through the Galaxy, we present a detailed analysis of the ground state
rotational lines of 16OH and 18OH in absorption towards the giant molecular
cloud complex, Sagittarius B2. We have modelled the line shapes to separate the
contribution of several galactic clouds along the line of sight and calculate
16OH/18OH ratios for each of these features. The best fitting values are in the
range 320-540, consistent with the previous measurements in the Galactic Disk
but slightly higher than the standard ratio in the Galactic Centre. They do not
show clear evidence for a gradient in the isotopic ratio with galactocentric
distance. The individual 16OH column densities relative to water give ratios of
[H2O/OH]=0.6-1.2, similar in magnitude to galactic clouds in the sight lines
towards W51 and W49. A comparison with CH indicates [OH/CH] ratios higher than
has been previously observed in diffuse clouds. We estimate OH abundances of
10^-7 - 10^-6 in the line of sight features.Comment: 10 pages, 6 figures, accepted for publication in A&
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