arXiv subjects

P. G. Martin

Publications and source records attributed to P. G. Martin.

At least 73 records · Page 4Linked to original sources

arXiv2011

The Mipsgal View of Supernova Remnants in the Galactic Plane

We report the detection of Galactic supernova remnants (SNRs) in the mid-infrared (at 24 and 70 μm), in the coordinate ranges 10 < l < 65 deg and 285 < l < 350 deg, |b| < 1 deg, using the Multiband Imaging Photometer (MIPS) aboard the Spitzer Space Telescope. We search for infrared counterparts to SNRs in Green's catalog and identify 39 out of 121, i.e., a detection rate of about 32%. Such a relatively low detection fraction is mainly due to confusion with nearby foreground/background sources and diffuse emission. The SNRs in our sample show a linear trend in [F8/F24] versus [F70/F24]. We compare their infrared fluxes with their corresponding radio flux at 1.4 GHz and find that most remnants have ratios of 70 μm to 1.4 GHz characteristic of SNRs (with the exception of a few which have ratios closer to those of H II regions). Furthermore, we retrieve a slope close to unity when correlating infrared (24 and 70 μm) with 1.4 GHz emission. Our survey is more successful in detecting remnants with bright X-ray emission, which we find is well correlated with the 24 μm morphology. Moreover, by comparing the power emitted in X-ray, infrared and radio, we conclude that the energy released in the infrared is comparable to the cooling in the X-ray range.

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arXiv2011

Herschel observations of Extra-Ordinary Sources: Methanol as a probe of physical conditions in Orion KL

We have examined methanol emission from Orion KL with of the {\em Herschel}/HIFI instrument, and detected two methanol bands centered at 524 GHz and 1061 GHz. The 524 GHz methanol band (observed in HIFI band 1a) is dominated by the isolated $Δ$J$=$0, K$=-4\rightarrow$-3, v$_t$$=$0 Q branch, and includes 25 E-type and 2 A-type transitions. The 1061 GHz methanol band (observed in HIFI band 4b) is dominated by the $Δ$J$=$0, K$=7\rightarrow$6, v$_t$$=$0 Q branch transitions which are mostly blended. We have used the isolated E-type v$_t$$=$0 methanol transitions to explore the physical conditions in the molecular gas. With HIFI's high velocity resolution, the methanol emission contributed by different spatial components along the line of sight toward Orion KL (hot core, low velocity flow, and compact ridge) can be distinguished and studied separately. The isolated transitions detected in these bands cover a broad energy range (upper state energy ranging from 80 K to 900 K), which provides a unique probe of the thermal structure in each spatial component. The observations further show that the compact ridge is externally heated. These observations demonstrate the power of methanol lines as probes of the physical conditions in warm regions in close proximity to young stars.

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arXiv2010

Dust in the diffuse emission of the galactic plane - The Herschel/Spitzer SED fitting

The first Herschel Hi-Gal images of the galactic plane unveil the far-infrared diffuse emission of the interstellar medium with an unprecedented angular resolution and sensitivity. In this paper, we present the first analysis of these data in combination with that of Spitzer Glimpse & Mipsgal. We selected a relatively diffuse and low excitation region of the l~59\,^{\circ} Hi-Gal Science Demonstration Phase field to perform a pixel by pixel fitting of the 8 to 500 microns SED using the DustEM dust emission model. We derived maps of the Very Small Grains (VSG) and PAH abundances from the model. Our analysis allows us to illustrate that the Aromatic Infrared Bands (AIB) intensity does not trace necessarily the PAH abundance but rather the product of "abundance x column density x intensity of the exciting radiation field". We show that the spatial structure of PACS70microns map resembles the shorter wavelengths (e.g. IRAC8microns) maps, because they trace both the intensity of exciting radiation field and column density. We also show that the modeled VSG contribution to PACS70microns (PACS160microns) band intensity can be up to 50% (7%). The interpretation of diffuse emission spectra at these wavelengths must take stochastically heated particles into account. Finally, this preliminary study emphasizes the potential of analyzing the full dust SED sampled by Herschel and Spitzer data, with a physical dust model (DustEM) to reach the properties of the dust at simultaneously large and small scales.

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arXiv2010

Detection of OH$^+$ and H$_2$O$^+$ towards Orion~KL

We report observations of the reactive molecular ions OH$^+$, H$_2$O$^+$, and H$_3$O$^+$ towards Orion KL with Herschel/HIFI. All three $N=1-0$ fine-structure transitions of OH$^+$ at 909, 971, and 1033GHz and both fine-structure components of the doublet {\it ortho}-H$_2$O$^+$ $1_{11}-0_{00}$ transition at 1115 and 1139GHz were detected; an upper limit was obtained for H$_3$O$^+$. OH$^+$ and H$_2$O$^+$ are observed purely in absorption, showing a narrow component at the source velocity of 9 kms$^{-1}$, and a broad blueshifted absorption similar to that reported recently for HF and {\it para}-H$_{2}^{18}$O, and attributed to the low velocity outflow of Orion KL. We estimate column densities of OH$^+$ and H$_2$O$^+$ for the 9 km s$^{-1}$ component of $9 \pm 3 \times 10^{12}$cm$^{-2}$ and $7 \pm 2 \times 10^{12}$cm$^{-2}$, and those in the outflow of $1.9 \pm 0.7 \times 10^{13}$cm$^{-2}$ and $1.0 \pm 0.3 \times 10^{13}$cm$^{-2}$. Upper limits of $2.4\times 10^{12}$cm$^{-2}$ and $8.7\times 10^{12}$cm$^{-2}$ were derived for the column densities of {\it ortho} and {\it para}-H$_3$O$^+$ from transitions near 985 and 1657GHz. The column densities of the three ions are up to an order of magnitude lower than those obtained from recent observations of W31C and W49N. The comparatively low column densities may be explained by a higher gas density despite the assumption of a very high ionization rate.

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arXiv2010

Targeted deep surveys of high Galactic latitude HI with the GBT

Over 800 sq. deg. of high Galactic latitude sky have been mapped at 21 cm with the Robert C. Byrd Green Bank Telescope (GBT). An improved knowledge of the telescope's beam characteristics has allowed us to reliably map not only regions of high column density, but also such regions as ELAIS N1, a targeted Spitzer field, which have very low HI column density. The additional fields we have observed cover a cross-section of dynamically and chemically interesting regions as indicated by the presence of intermediate/high velocity gas and/or anomalous far-IR (dust) colour.

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arXiv2010

Gas morphology and energetics at the surface of PDRs: new insights with Herschel observations of NGC 7023

We investigate the physics and chemistry of the gas and dust in dense photon-dominated regions (PDRs), along with their dependence on the illuminating UV field. Using Herschel-HIFI observations, we study the gas energetics in NGC 7023 in relation to the morphology of this nebula. NGC 7023 is the prototype of a PDR illuminated by a B2V star and is one of the key targets of Herschel. Our approach consists in determining the energetics of the region by combining the information carried by the mid-IR spectrum (extinction by classical grains, emission from very small dust particles) with that of the main gas coolant lines. In this letter, we discuss more specifically the intensity and line profile of the 158 micron (1901 GHz) [CII] line measured by HIFI and provide information on the emitting gas. We show that both the [CII] emission and the mid-IR emission from polycyclic aromatic hydrocarbons (PAHs) arise from the regions located in the transition zone between atomic and molecular gas. Using the Meudon PDR code and a simple transfer model, we find good agreement between the calculated and observed [CII] intensities. HIFI observations of NGC 7023 provide the opportunity to constrain the energetics at the surface of PDRs. Future work will include analysis of the main coolant line [OI] and use of a new PDR model that includes PAH-related species.

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arXiv2010

Herschel observations of EXtra-Ordinary Sources (HEXOS): Observations of H2O and its isotopologues towards Orion KL

We report the detection of more than 48 velocity-resolved ground rotational state transitions of H2(16)O, H2(18)O, and H2(17)O - most for the first time - in both emission and absorption toward Orion KL using Herschel/HIFI. We show that a simple fit, constrained to match the known emission and absorption components along the line of sight, is in excellent agreement with the spectral profiles of all the water lines. Using the measured H2(18)O line fluxes, which are less affected by line opacity than their H2(16)O counterparts, and an escape probability method, the column densities of H2(18)O associated with each emission component are derived. We infer total water abundances of 7.4E-5, 1.0E-5, and 1.6E-5 for the plateau, hot core, and extended warm gas, respectively. In the case of the plateau, this value is consistent with previous measures of the Orion-KL water abundance as well as those of other molecular outflows. In the case of the hot core and extended warm gas, these values are somewhat higher than water abundances derived for other quiescent clouds, suggesting that these regions are likely experiencing enhanced water-ice sublimation from (and reduced freeze-out onto) grain surfaces due to the warmer dust in these sources.

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arXiv2010

Reversal of infall in SgrB2(M) revealed by Herschel/HIFI observations of HCN lines at THz frequencies

To investigate the accretion and feedback processes in massive star formation, we analyze the shapes of emission lines from hot molecular cores, whose asymmetries trace infall and expansion motions. The high-mass star forming region SgrB2(M) was observed with Herschel/HIFI (HEXOS key project) in various lines of HCN and its isotopologues, complemented by APEX data. The observations are compared to spherically symmetric, centrally heated models with density power-law gradient and different velocity fields (infall or infall+expansion), using the radiative transfer code RATRAN. The HCN line profiles are asymmetric, with the emission peak shifting from blue to red with increasing J and decreasing line opacity (HCN to H$^{13}$CN). This is most evident in the HCN 12--11 line at 1062 GHz. These line shapes are reproduced by a model whose velocity field changes from infall in the outer part to expansion in the inner part. The qualitative reproduction of the HCN lines suggests that infall dominates in the colder, outer regions, but expansion dominates in the warmer, inner regions. We are thus witnessing the onset of feedback in massive star formation, starting to reverse the infall and finally disrupting the whole molecular cloud. To obtain our result, the THz lines uniquely covered by HIFI were critically important.

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arXiv2010

Herschel observations of deuterated water towards Sgr B2(M)

Observations of HDO are an important complement for studies of water, because they give strong constraints on the formation processes -- grain surfaces versus energetic process in the gas phase, e.g. in shocks. The HIFI observations of multiple transitions of HDO in Sgr~B2(M) presented here allow the determination of the HDO abundance throughout the envelope, which has not been possible before with ground-based observations only. The abundance structure has been modeled with the spherical Monte Carlo radiative transfer code RATRAN, which also takes radiative pumping by continuum emission from dust into account. The modeling reveals that the abundance of HDO rises steeply with temperature from a low abundance ($2.5\times 10^{-11}$) in the outer envelope at temperatures below 100~K through a medium abundance ($1.5\times 10^{-9}$) in the inner envelope/outer core, at temperatures between 100 and 200~K, and finally a high abundance ($3.5\times 10^{-9}$) at temperatures above 200~K in the hot core.

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arXiv2010

Herschel observations of EXtra-Ordinary Sources: The Terahertz spectrum of Orion KL seen at high spectral resolution

We present the first high spectral resolution observations of Orion KL in the frequency ranges 1573.4 - 1702.8 GHz (band 6b) and 1788.4 - 1906.8 GHz (band 7b) obtained using the HIFI instrument on board the Herschel Space Observatory. We characterize the main emission lines found in the spectrum, which primarily arise from a range of components associated with Orion KL including the hot core, but also see widespread emission from components associated with molecular outflows traced by H2O, SO2, and OH. We find that the density of observed emission lines is significantly diminished in these bands compared to lower frequency Herschel/HIFI bands.

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arXiv2010

Herschel observations of extra-ordinary sources: Detection of Hydrogen Fluoride in absorption towards Orion~KL

We report a detection of the fundamental rotational transition of hydrogen fluoride in absorption towards Orion KL using Herschel/HIFI. After the removal of contaminating features associated with common molecules ("weeds"), the HF spectrum shows a P-Cygni profile, with weak redshifted emission and strong blue-shifted absorption, associated with the low-velocity molecular outflow. We derive an estimate of 2.9 x 10^13 cm^-2 for the HF column density responsible for the broad absorption component. Using our best estimate of the H2 column density within the low-velocity molecular outflow, we obtain a lower limit of ~1.6 x 10^-10 for the HF abundance relative to hydrogen nuclei, corresponding to 0.6% of the solar abundance of fluorine. This value is close to that inferred from previous ISO observations of HF J=2--1 absorption towards Sgr B2, but is in sharp contrast to the lower limit of 6 x 10^-9 derived by Neufeld et al. (2010) for cold, foreground clouds on the line of sight towards G10.6-0.4.

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arXiv2010

Herschel observations of EXtra-Ordinary Sources: the present and future of spectral surveys with Herschel/HIFI

We present initial results from the Herschel GT key program: Herschel observations of EXtra-Ordinary Sources (HEXOS) and outline the promise and potential of spectral surveys with Herschel/HIFI. The HIFI instrument offers unprecedented sensitivity, as well as continuous spectral coverage across the gaps imposed by the atmosphere, opening up a largely unexplored wavelength regime to high-resolution spectroscopy. We show the spectrum of Orion KL between 480 and 560 GHz and from 1.06 to 1.115 THz. From these data, we confirm that HIFI separately measures the dust continuum and spectrally resolves emission lines in Orion KL. Based on this capability we demonstrate that the line contribution to the broad-band continuum in this molecule-rich source is ~20-40% below 1 THz and declines to a few percent at higher frequencies. We also tentatively identify multiple transitions of HD18O in the spectra. The first detection of this rare isotopologue in the interstellar medium suggests that HDO emission is optically thick in the Orion hot core with HDO/H2O ~ 0.02. We discuss the implications of this detection for the water D/H ratio in hot cores.

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arXiv2010

Herschel observations of extra-ordinary sources: Detecting spiral arm clouds by CH absorption lines

We have observed CH absorption lines ($J=3/2, N=1 \leftarrow J=1/2, N=1$) against the continuum source Sgr~B2(M) using the \textit{Herschel}/HIFI instrument. With the high spectral resolution and wide velocity coverage provided by HIFI, 31 CH absorption features with different radial velocities and line widths are detected and identified. The narrower line width and lower column density clouds show `spiral arm' cloud characteristics, while the absorption component with the broadest line width and highest column density corresponds to the gas from the Sgr~B2 envelope. The observations show that each `spiral arm' harbors multiple velocity components, indicating that the clouds are not uniform and that they have internal structure. This line-of-sight through almost the entire Galaxy offers unique possibilities to study the basic chemistry of simple molecules in diffuse clouds, as a variety of different cloud classes are sampled simultaneously. We find that the linear relationship between CH and H$_2$ column densities found at lower $A_V$ by UV observations does not continue into the range of higher visual extinction. There, the curve flattens, which probably means that CH is depleted in the denser cores of these clouds.

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arXiv2010

Herschel observations in the ultracompact HII region Mon R2: Water in dense Photon-dominated regions (PDRs)

Mon R2, at a distance of 830 pc, is the only ultracompact HII region (UC HII) where the photon-dominated region (PDR) between the ionized gas and the molecular cloud can be resolved with Herschel. HIFI observations of the abundant compounds 13CO, C18O, o-H2-18O, HCO+, CS, CH, and NH have been used to derive the physical and chemical conditions in the PDR, in particular the water abundance. The 13CO, C18O, o-H2-18O, HCO+ and CS observations are well described assuming that the emission is coming from a dense (n=5E6 cm-3, N(H2)>1E22 cm-2) layer of molecular gas around the UC HII. Based on our o-H2-18O observations, we estimate an o-H2O abundance of ~2E-8. This is the average ortho-water abundance in the PDR. Additional H2-18O and/or water lines are required to derive the water abundance profile. A lower density envelope (n~1E5 cm-3, N(H2)=2-5E22 cm-2) is responsible for the absorption in the NH 1_1-0_2 line. The emission of the CH ground state triplet is coming from both regions with a complex and self-absorbed profile in the main component. The radiative transfer modeling shows that the 13CO and HCO+ line profiles are consistent with an expansion of the molecular gas with a velocity law, v_e =0.5 x (r/Rout)^{-1} km/s, although the expansion velocity is poorly constrained by the observations presented here.

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arXiv2010

Herschel/HIFI discovery of interstellar chloronium (H$_2$Cl$^+$)

We report the first detection of chloronium, H$_2$Cl$^+$, in the interstellar medium, using the HIFI instrument aboard the \emph{Herschel} Space Observatory. The $2_{12}-1_{01}$ lines of ortho-H$_2^{35}$Cl$^+$ and ortho-H$_2^{37}$Cl$^+$ are detected in absorption towards NGC~6334I, and the $1_{11}-0_{00}$ transition of para-H$_2^{35}$Cl$^+$ is detected in absorption towards NGC~6334I and Sgr~B2(S). The H$_2$Cl$^+$ column densities are compared to those of the chemically-related species HCl. The derived HCl/H$_2$Cl$^+$ column density ratios, $\sim$1--10, are within the range predicted by models of diffuse and dense Photon Dominated Regions (PDRs). However, the observed H$_2$Cl$^+$ column densities, in excess of $10^{13}$~cm$^{-2}$, are significantly higher than the model predictions. Our observations demonstrate the outstanding spectroscopic capabilities of HIFI for detecting new interstellar molecules and providing key constraints for astrochemical models.

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arXiv2010

Herschel/HIFI measurements of the ortho/para ratio in water towards Sagittarius B2(M) and W31C

We present Herschel/HIFI observations of the fundamental rotational transitions of ortho- and para-H$_2^{16}$O and H$_2^{18}$O in absorption towards Sagittarius~B2(M) and W31C. The ortho/para ratio in water in the foreground clouds on the line of sight towards these bright continuum sources is generally consistent with the statistical high-temperature ratio of 3, within the observational uncertainties. However, somewhat unexpectedly, we derive a low ortho/para ratio of $2.35 \pm 0.35$, corresponding to a spin temperature of $\sim$27~K, towards Sagittarius~B2(M) at velocities of the expanding molecular ring. Water molecules in this region appear to have formed with, or relaxed to, an ortho/para ratio close to the value corresponding to the local temperature of the gas and dust.

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arXiv2010

Herschel observations of ortho- and para-oxidaniumyl (H2O+) in spiral arm clouds toward Sgr B2(M)

H2O+ has been observed in its ortho- and para- states toward the massive star forming core Sgr B2(M), located close to the Galactic center. The observations show absorption in all spiral arm clouds between the Sun and Sgr B2. The average o/p ratio of H2O+ in most velocity intervals is 4.8, which corresponds to a nuclear spin temperature of 21 K. The relationship of this spin temperature to the formation temperature and current physical temperature of the gas hosting H2O+ is discussed, but no firm conclusion is reached. In the velocity interval 0 to 60 km/s, an ortho/para ratio of below unity is found, but if this is due to an artifact of contamination by other species or real is not clear.

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arXiv2010

Herschel-SPIRE observations of the Polaris flare : structure of the diffuse interstellar medium at the sub-parsec scale

We present a power spectrum analysis of the Herschel-SPIRE observations of the Polaris flare, a high Galactic latitude cirrus cloud midway between the diffuse and molecular phases. The SPIRE images of the Polaris flare reveal for the first time the structure of the diffuse interstellar medium down to 0.01 parsec over a 10 square degrees region. These exceptional observations highlight the highly filamentary and clumpy structure of the interstellar medium even in diffuse regions of the map. The power spectrum analysis shows that the structure of the interstellar medium is well described by a single power law with an exponent of -2.7 +- 0.1 at all scales from 30" to 8 degrees. That the power spectrum slope of the dust emission is constant down to the SPIRE angular resolution is an indication that the inertial range of turbulence extends down to the 0.01 pc scale. The power spectrum analysis also allows the identification of a Poissonian component at sub-arcminute scales in agreement with predictions of the cosmic infrared background level at SPIRE wavelengths. Finally, the comparison of the SPIRE and IRAS 100 micron data of the Polaris flare clearly assesses the capability of SPIRE in maping diffuse emission over large areas.

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