602 research outputs found
New accurate measurement of 36ArH+ and 38ArH+ ro-vibrational transitions by high resolution IR absorption spectroscopy
The protonated Argon ion, ArH, has been identified recently in
the Crab Nebula (Barlow et al. 2013) from Herschel spectra. Given the
atmospheric opacity at the frequency of its =1-0 and =2-1 rotational
transitions (617.5 and 1234.6 GHz, respectively), and the current lack of
appropriate space observatories after the recent end of the Herschel mission,
future studies on this molecule will rely on mid-infrared observations. We
report on accurate wavenumber measurements of ArH and
ArH rotation-vibration transitions in the =1-0 band in the
range 4.1-3.7 m (2450-2715 cm). The wavenumbers of the (0)
transitions of the =1-0 band are 2612.501350.00033 and
2610.701770.00042 cm () for ArH and
ArH, respectively. The calculated opacity for a gas thermalized at
a temperature of 100 K and a linewidth of 1 km.s of the (0) line is
(ArH). For column densities of
ArH larger than cm, significant absorption
by the (0) line can be expected against bright mid-IR sources
In-flight calibration of the Herschel-SPIRE instrument
SPIRE, the Spectral and Photometric Imaging REceiver, is the Herschel Space Observatory's submillimetre camera and spectrometer. It contains a three-band imaging photometer operating at 250, 350 and 500 μm, and an imaging Fourier-transform spectrometer (FTS) covering 194–671 μm (447-1550 GHz). In this paper we describe the initial approach taken to the absolute calibration of the SPIRE instrument using a combination of the emission from the Herschel telescope itself and the modelled continuum emission from solar system objects and other astronomical targets. We present the photometric, spectroscopic and spatial accuracy that is obtainable in data processed through the “standard” pipelines. The overall photometric accuracy at this stage of the mission is estimated as 15% for the photometer and between 15 and 50% for the spectrometer. However, there remain issues with the photometric accuracy of the spectra of low flux sources in the longest wavelength part of the SPIRE spectrometer band. The spectrometer wavelength accuracy is determined to be better than 1/10th of the line FWHM. The astrometric accuracy in SPIRE maps is found to be 2 arcsec when the latest calibration data are used. The photometric calibration of the SPIRE instrument is currently determined by a combination of uncertainties in the model spectra of the astronomical standards and the data processing methods employed for map and spectrum calibration. Improvements in processing techniques and a better understanding of the instrument performance will lead to the final calibration accuracy of SPIRE being determined only by uncertainties in the models of astronomical standards
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
The dust envelope of the pre-planetary nebula IRAS19475+3119
We present the spectral energy distribution (SED) of the pre-planetary
nebula, IRAS 19475+3119 (I19475), from the optical to the far-infrared. We
identify emission features due to crystalline silicates in the ISO SWS spectra
of the star. We have fitted the SED of I19475 using a 1-D radiative transfer
code, and find that a shell with inner and outer radii of 8.8X10^{16} and
4.4X10^{17}cm, and dust temperatures ranging from about 94K to 46K provide the
best fit. The mass of this shell is greater than/equal to
1[34cm^{2}g^{-1}/kappa(100micron)][delta/200]M_Sun, where kappa(100micron) is
the 100micron dust mass absorption coefficient (per unit dust mass), and delta
is the gas-to-dust ratio. In agreement with results from optical imaging and
millimeter-wave observations of CO emission of I19475, our model fits support
an r^{-3} density law for its dust shell, with important implications for the
interaction process between the fast collimated post-AGB winds and the dense
AGB envelopes which results in the observed shapes of PPNs and PNs. We find
that the observed JCMT flux at sub-millimeter wavelengths (850micron) is a
factor ~ 2 larger than our model flux, suggesting the presence of large dust
grains in the dust shell of I19475 which are not accounted for by our adopted
standard MRN grain size distribution.Comment: 38 pages, 8 figures. Accepted for publication in Ap
OH/IR stars and their superwinds as observed by the Herschel Space Observatory
Aim : In order to study the history of mass loss in extreme OH/IR stars, we
observed a number of these objects using CO as a tracer of the density and
temperature structure of their circumstellar envelopes.
Method : Combining CO observations from the Herschel Space Observatory with
those from the ground, we trace mass loss rates as a function of radius in five
extreme OH/IR stars. Using radiative transfer modelling, we modelled the dusty
envelope as well as the CO emission. The high-rotational transitions of CO
indicate that they originate in a dense superwind region close to the star
while the lower transitions tend to come from a more tenuous outer wind which
is a result of the mass loss since the early AGB phase.
Result : The models of the circumstellar envelopes around these stars suggest
that they have entered a superwind phase in the past 200 - 500 years. The low
18O/17O (~ 0.1 compared to the solar abundance ratio of ~ 5) and 12C/13C (3-30
cf. the solar value of 89) ratios derived from our study support the idea that
these objects have undergone hot-bottom burning and hence that they are massive
M >= 5 solar-mass AGB stars.Comment: 10 pages with 11 figures Accepted for publication by Astronomy &
Astrophysic
The dust mass in Cassiopeia A from a spatially resolved Herschel analysis
Theoretical models predict that core-collapse supernovae (CCSNe) can be efficient dust producers (0.1–1.0 M⊙), potentially accounting for most of the dust production in the early Universe. Observational evidence for this dust production efficiency is however currently limited to only a few CCSN remnants (e.g. SN 1987A, Crab nebula). In this paper, we revisit the dust mass produced in Cassiopeia A (Cas A), a ∼330-yr old O-rich Galactic supernova remnant (SNR) embedded in a dense interstellar foreground and background. We present the first spatially resolved analysis of Cas A based on Spitzer and Herschel infrared and submillimetre data at a common resolution of ∼0.6 arcmin for this 5 arcmin diameter remnant following a careful removal of contaminating line emission and synchrotron radiation. We fit the dust continuum from 17 to 500 μm with a four-component interstellar medium and supernova (SN) dust model. We find a concentration of cold dust in the unshocked ejecta of Cas A and derive a mass of 0.3–0.5 M⊙ of silicate grains freshly produced in the SNR, with a lower limit of ≥0.1–0.2 M⊙. For a mixture of 50 per cent of silicate-type grains and 50 per cent of carbonaceous grains, we derive a total SN dust mass between 0.4 and 0.6 M⊙. These dust mass estimates are higher than from most previous studies of Cas A and support the scenario of SN-dominated dust production at high redshifts. We furthermore derive an interstellar extinction map for the field around Cas A which towards Cas A gives average values of AV = 6–8 mag, up to a maximum of AV = 15 mag
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