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

New accurate measurement of 36ArH+ and 38ArH+ ro-vibrational transitions by high resolution IR absorption spectroscopy

The protonated Argon ion, $^{36}$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 $J$=1-0 and $J$=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 $^{36}$ArH$^{+}$ and $^{38}$ArH$^{+}$ rotation-vibration transitions in the $v$=1-0 band in the range 4.1-3.7 $\mu$m (2450-2715 cm$^{-1}$). The wavenumbers of the $R$(0) transitions of the $v$=1-0 band are 2612.50135$\pm$0.00033 and 2610.70177$\pm$0.00042 cm$^{-1}$ ($\pm3\sigma$) for $^{36}$ArH$^{+}$ and $^{38}$ArH$^{+}$, respectively. The calculated opacity for a gas thermalized at a temperature of 100 K and a linewidth of 1 km.s$^{-1}$ of the $R$(0) line is $1.6\times10^{-15}\times N$($^{36}$ArH$^+$). For column densities of $^{36}$ArH$^+$ larger than $1\times 10^{13}$ cm$^{-2}$, significant absorption by the $R$(0) line can be expected against bright mid-IR sources

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

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