N. Billot - Academia.edu (original) (raw)

Papers by N. Billot

Experimental Astronomy, 2013

The absolute stability of the PACS bolometer response over the entire mission lifetime without ap... more The absolute stability of the PACS bolometer response over the entire mission lifetime without applying any corrections is about 0.5% (standard deviation) or about 8% peak-to-peak. This fantastic stability allows us to calibrate all scientific measurements by a fixed and time-independent response file, without using any information from the PACS internal calibration sources. However, the analysis of calibration block observations revealed clear correlations of the internal source signals with the evaporator temperature and a signal drift during the first half hour after the cooler recycling. These effects are small, but can be seen in repeated measurements of standard stars. From our analysis we established corrections for both effects which push the stability of the PACS bolometer response to about 0.2% (stdev) or 2% in the blue, 3% in the green and 5% in the red channel (peak-to-peak). After both corrections we still see a correlation of the signals with PACS FPU temperatures, possibly caused by parasitic heat influences via the Kevlar wires which connect the bolometers with the PACS Focal Plane Unit. No aging effect or degradation of the photometric system during the mission lifetime has been found.

We present the first results from the science demonstration phase for the Hi-GAL survey, the Hers... more We present the first results from the science demonstration phase for the Hi-GAL survey, the Herschel key program that will map the inner Galactic plane of the Milky Way in 5 bands. We outline our data reduction strategy and present some science highlights on the two observed 2 • × 2 • tiles approximately centered at l = 30 • and l = 59 •. The two regions are extremely rich in intense and highly structured extended emission which shows a widespread organization in filaments. Source SEDs can be built for hundreds of objects in the two fields, and physical parameters can be extracted, for a good fraction of them where the distance could be estimated. The compact sources (which we will call cores' in the following) are found for the most part to be associated with the filaments, and the relationship to the local beam-averaged column density of the filament itself shows that a core seems to appear when a threshold around A V ∼ 1 is exceeded for the regions in the l = 59 • field; a A V value between 5 and 10 is found for the l = 30 • field, likely due to the relatively higher distances of the sources. This outlines an exciting scenario where diffuse clouds first collapse into filaments, which later fragment to cores where the column density has reached a critical level. In spite of core L/M ratios being well in excess of a few for many sources, we find core surface densities between 0.03 and 0.5 g cm −2. Our results are in good agreement with recent MHD numerical simulations of filaments forming from large-scale converging flows.

Experimental Astronomy, 2014

Revue de Chirurgie Orthopédique et Traumatologique, 2012

We present a catalog of compact sources detected in the Spitzer/MIPSGAL 24 microns survey data. T... more We present a catalog of compact sources detected in the Spitzer/MIPSGAL 24 microns survey data. These small (< 1 arcminute) rings, bubbles, disks or shells are pervasive through the entire Galactic plane in the mid-infrared. Over 400 such sources are detected from visual inspection of the MIPSGAL mosaic images. Their average density is found to be around 1.5 bubbles per square degree. We identify 10% of these objects by extensive cross matching with available catalogs. We find that the majority are planetary nebulae, with several additional supernova remnants, post-AGB star, Wolf-Rayet and Luminous Blue Variables. The remaining 90% of the bubbles are yet unknown objects. Most of them are detected at 24 microns but neither at 8 nor 70 microns. We also present Spitzer/IRS high-resolution spectroscopic observations of 4 objects obtained as part of the Director's Discretionary Time Program. Their spectra show significant variations in the low to high excitation gas lines ratio, as well as in the dust continuum intensity, suggesting some are dust free objects. We compare these spectroscopic data to templates of evolved stars, including PNe, SNRs and extremely rare and massive luminous blue variables to constrain their true nature.

The Astrophysical Journal, 2011

We use a Minimum Spanning Tree algorithm to characterize the spatial distribution of Galactic Far... more We use a Minimum Spanning Tree algorithm to characterize the spatial distribution of Galactic Far-IR sources and derive their clustering properties. We aim to reveal the spatial imprint of different types of star forming processes, e.g. isolated spontaneous fragmentation of dense molecular clouds, or events of triggered star formation around Hii regions, and highlight global properties of star formation in the Galaxy. We plan to exploit the entire Hi-GAL survey of the inner Galactic plane to gather significant statistics on the clustering properties of star forming regions, and to look for possible correlations with source properties such as mass, temperature or evolutionary stage. In this paper we present a pilot study based on the two 2 • ×2 • fields centered at longitudes l = 30 • and l = 59 • obtained during the Science Demonstration Phase (SDP) of the Herschel mission. We find that over half of the clustered sources are associated with Hii regions and infrared dark clouds. Our analysis also reveals a smooth chromatic evolution of the spatial distribution where sources detected at short-wavelengths, likely proto-stars surrounded by warm circumstellar material emitting in the far-infrared, tend to be clustered in dense and compact groups around Hii regions while sources detected at long-wavelengths, presumably cold and dusty density enhancements of the ISM emitting in the sub-millimeter, are distributed in larger and looser groups.

The Astronomical Journal, 2010

We present a catalog of 416 extended, resolved, disk-and ring-like objects as detected in the MIP... more We present a catalog of 416 extended, resolved, disk-and ring-like objects as detected in the MIPSGAL 24 µm survey of the Galactic plane. This catalog is the result of a search in the MIPSGAL image data for generally circularly symmetric, extended "bubbles" without prior knowledge or expectation of their physical nature. Most of the objects have no extended counterpart at 8 or 70 µm, with less than 20% detections at each wavelength. For the 54 objects with central point sources, the sources are nearly always seen in all IRAC bands. About 70 objects (16%) have been previously identified, with another 35 listed as IRAS sources. Among the identified objects, those with central sources are mostly listed as emission-line stars, but with other source types including supernova remnants, luminous blue variables, and planetary nebulae. The 57 identified objects (of 362) without central sources are nearly all PNe (∼90%), which suggests that a large fraction of the 300+ unidentified objects in this category are also PNe. These identifications suggest that this is primarily a catalog of evolved stars. Also included in the catalog are two filamentary objects that are almost certainly supernova remnants, and ten unusual compact extended objects discovered in the search. Two of these show remarkable spiral structure at both 8 and 24 µm. These are likely background galaxies previously hidden by the intervening Galactic plane.

The Astrophysical Journal, 2012

We have obtained time series observations of the Orion Nebula Cluster at 70 μm and 160 μm from th... more We have obtained time series observations of the Orion Nebula Cluster at 70 μm and 160 μm from the Herschel/PACS Photometer. This represents the first wide-field far-infrared photometric monitoring of a young star-forming region. The acquired 35 × 35 maps show complex extended structures, with unprecedented detail, that trace the interaction between the molecular gas and the young hot stars. We detect 43 protostars, most of which are situated along the integral-shaped filament extending from the Orion nebula, through OMC 2 and OMC 3. We present high-reliability light curves for some of these objects using the first six epochs of our observing program spread over 6 weeks. We find amplitude variations in excess of 20% for a fraction of the detected protostars over periods as short as a few weeks. This is inconsistent with the dynamical timescales of cool far-IR emitting material that orbits at hundreds of AU from the protostar, and it suggests that the mechanism(s) responsible for the observed variability originates from the inner region of the protostars, likely driven by variable mass accretion.

Journal of Low Temperature Physics, 2013

NIKA (New IRAM KID Arrays) is a dual-band imaging instrument installed at the IRAM (Institut de R... more NIKA (New IRAM KID Arrays) is a dual-band imaging instrument installed at the IRAM (Institut de RadioAstronomie Millimetrique) 30-meter telescope at Pico Veleta (Spain). Two distinct Kinetic Inductance Detectors (KID) focal planes allow the camera to simultaneaously image a field-of-view of about 2 arc-min in the bands 125 to 175 GHz (150 GHz) and 200 to 280 GHz (240 GHz). The sensitivity and stability achieved during the last commissioning Run in June 2013 allows opening the instrument to general observers. We report here the latest results, in particular in terms of sensitivity, now comparable to the state-of-theart Transition Edge Sensors (TES) bolometers, relative and absolute photometry. We describe briefly the next generation NIKA-2 instrument, selected by IRAM to occupy, from 2015, the continuum imager/polarimeter slot at the 30-m telescope.

Astronomy and Astrophysics, 2010

Context. It has been shown that by means of different physical mechanisms the expansion of H ii r... more Context. It has been shown that by means of different physical mechanisms the expansion of H ii regions can trigger the formation of new stars of all masses. This process may be important to the formation of massive stars but has never been quantified in the Galaxy. Aims. We use Herschel-PACS and -SPIRE images from the Herschel Infrared survey of the Galactic plane, Hi-GAL, to perform this study. Methods. We combine the Spitzer-GLIMPSE and -MIPSGAL, radio-continuum and sub-millimeter surveys such as ATLASGAL with Hi-GAL to study Young Stellar Objects (YSOs) observed towards Galactic H ii regions. We select a representative H ii region, N49, located in the field centered on l=30 • observed as part of the Hi-GAL Science Demonstration Phase, to demonstrate the importance Hi-GAL will have to this field of research.

Astronomy and Astrophysics, 2010

The Photodetector Array Camera and Spectrometer (PACS) is one of the three science instruments on... more The Photodetector Array Camera and Spectrometer (PACS) is one of the three science instruments on ESA's far infrared and submillimetre observatory. It employs two Ge:Ga photoconductor arrays (stressed and unstressed) with 16 × 25 pixels, each, and two filled silicon bolometer arrays with 16 × 32 and 32 × 64 pixels, respectively, to perform integral-field spectroscopy and imaging photometry in the 60-210 µm wavelength regime. In photometry mode, it simultaneously images two bands, 60-85 µm or 85-125 µm and 125-210 µm, over a field of view of ∼ 1.75 × 3.5 , with close to Nyquist beam sampling in each band. In spectroscopy mode, it images a field of 47 × 47 , resolved into 5 × 5 pixels, with an instantaneous spectral coverage of ∼ 1500 km/s and a spectral resolution of ∼ 175 km/s. We summarise the design of the instrument, describe observing modes, calibration, and data analysis methods, and present our current assessment of the in-orbit performance of the instrument based on the Performance Verification tests. PACS is fully operational, and the achieved performance is close to or better than the pre-launch predictions.

Astronomy and Astrophysics, 2010

Abstract The Photodetector Array Camera and Spectrometer (PACS) is one of the three science instr... more Abstract The Photodetector Array Camera and Spectrometer (PACS) is one of the three science instruments on ESA's far infrared and submillimetre observatory. It employs two Ge:Ga photoconductor arrays (stressed and unstressed) with pixels, each, and two filled silicon bolometer arrays ...

Experimental Astronomy, 2013

The absolute stability of the PACS bolometer response over the entire mission lifetime without ap... more The absolute stability of the PACS bolometer response over the entire mission lifetime without applying any corrections is about 0.5% (standard deviation) or about 8% peak-to-peak. This fantastic stability allows us to calibrate all scientific measurements by a fixed and time-independent response file, without using any information from the PACS internal calibration sources. However, the analysis of calibration block observations revealed clear correlations of the internal source signals with the evaporator temperature and a signal drift during the first half hour after the cooler recycling. These effects are small, but can be seen in repeated measurements of standard stars. From our analysis we established corrections for both effects which push the stability of the PACS bolometer response to about 0.2% (stdev) or 2% in the blue, 3% in the green and 5% in the red channel (peak-to-peak). After both corrections we still see a correlation of the signals with PACS FPU temperatures, possibly caused by parasitic heat influences via the Kevlar wires which connect the bolometers with the PACS Focal Plane Unit. No aging effect or degradation of the photometric system during the mission lifetime has been found.

We present the first results from the science demonstration phase for the Hi-GAL survey, the Hers... more We present the first results from the science demonstration phase for the Hi-GAL survey, the Herschel key program that will map the inner Galactic plane of the Milky Way in 5 bands. We outline our data reduction strategy and present some science highlights on the two observed 2 • × 2 • tiles approximately centered at l = 30 • and l = 59 •. The two regions are extremely rich in intense and highly structured extended emission which shows a widespread organization in filaments. Source SEDs can be built for hundreds of objects in the two fields, and physical parameters can be extracted, for a good fraction of them where the distance could be estimated. The compact sources (which we will call cores' in the following) are found for the most part to be associated with the filaments, and the relationship to the local beam-averaged column density of the filament itself shows that a core seems to appear when a threshold around A V ∼ 1 is exceeded for the regions in the l = 59 • field; a A V value between 5 and 10 is found for the l = 30 • field, likely due to the relatively higher distances of the sources. This outlines an exciting scenario where diffuse clouds first collapse into filaments, which later fragment to cores where the column density has reached a critical level. In spite of core L/M ratios being well in excess of a few for many sources, we find core surface densities between 0.03 and 0.5 g cm −2. Our results are in good agreement with recent MHD numerical simulations of filaments forming from large-scale converging flows.

Experimental Astronomy, 2014

Revue de Chirurgie Orthopédique et Traumatologique, 2012

We present a catalog of compact sources detected in the Spitzer/MIPSGAL 24 microns survey data. T... more We present a catalog of compact sources detected in the Spitzer/MIPSGAL 24 microns survey data. These small (< 1 arcminute) rings, bubbles, disks or shells are pervasive through the entire Galactic plane in the mid-infrared. Over 400 such sources are detected from visual inspection of the MIPSGAL mosaic images. Their average density is found to be around 1.5 bubbles per square degree. We identify 10% of these objects by extensive cross matching with available catalogs. We find that the majority are planetary nebulae, with several additional supernova remnants, post-AGB star, Wolf-Rayet and Luminous Blue Variables. The remaining 90% of the bubbles are yet unknown objects. Most of them are detected at 24 microns but neither at 8 nor 70 microns. We also present Spitzer/IRS high-resolution spectroscopic observations of 4 objects obtained as part of the Director's Discretionary Time Program. Their spectra show significant variations in the low to high excitation gas lines ratio, as well as in the dust continuum intensity, suggesting some are dust free objects. We compare these spectroscopic data to templates of evolved stars, including PNe, SNRs and extremely rare and massive luminous blue variables to constrain their true nature.

The Astrophysical Journal, 2011

We use a Minimum Spanning Tree algorithm to characterize the spatial distribution of Galactic Far... more We use a Minimum Spanning Tree algorithm to characterize the spatial distribution of Galactic Far-IR sources and derive their clustering properties. We aim to reveal the spatial imprint of different types of star forming processes, e.g. isolated spontaneous fragmentation of dense molecular clouds, or events of triggered star formation around Hii regions, and highlight global properties of star formation in the Galaxy. We plan to exploit the entire Hi-GAL survey of the inner Galactic plane to gather significant statistics on the clustering properties of star forming regions, and to look for possible correlations with source properties such as mass, temperature or evolutionary stage. In this paper we present a pilot study based on the two 2 • ×2 • fields centered at longitudes l = 30 • and l = 59 • obtained during the Science Demonstration Phase (SDP) of the Herschel mission. We find that over half of the clustered sources are associated with Hii regions and infrared dark clouds. Our analysis also reveals a smooth chromatic evolution of the spatial distribution where sources detected at short-wavelengths, likely proto-stars surrounded by warm circumstellar material emitting in the far-infrared, tend to be clustered in dense and compact groups around Hii regions while sources detected at long-wavelengths, presumably cold and dusty density enhancements of the ISM emitting in the sub-millimeter, are distributed in larger and looser groups.

The Astronomical Journal, 2010

We present a catalog of 416 extended, resolved, disk-and ring-like objects as detected in the MIP... more We present a catalog of 416 extended, resolved, disk-and ring-like objects as detected in the MIPSGAL 24 µm survey of the Galactic plane. This catalog is the result of a search in the MIPSGAL image data for generally circularly symmetric, extended "bubbles" without prior knowledge or expectation of their physical nature. Most of the objects have no extended counterpart at 8 or 70 µm, with less than 20% detections at each wavelength. For the 54 objects with central point sources, the sources are nearly always seen in all IRAC bands. About 70 objects (16%) have been previously identified, with another 35 listed as IRAS sources. Among the identified objects, those with central sources are mostly listed as emission-line stars, but with other source types including supernova remnants, luminous blue variables, and planetary nebulae. The 57 identified objects (of 362) without central sources are nearly all PNe (∼90%), which suggests that a large fraction of the 300+ unidentified objects in this category are also PNe. These identifications suggest that this is primarily a catalog of evolved stars. Also included in the catalog are two filamentary objects that are almost certainly supernova remnants, and ten unusual compact extended objects discovered in the search. Two of these show remarkable spiral structure at both 8 and 24 µm. These are likely background galaxies previously hidden by the intervening Galactic plane.

The Astrophysical Journal, 2012

We have obtained time series observations of the Orion Nebula Cluster at 70 μm and 160 μm from th... more We have obtained time series observations of the Orion Nebula Cluster at 70 μm and 160 μm from the Herschel/PACS Photometer. This represents the first wide-field far-infrared photometric monitoring of a young star-forming region. The acquired 35 × 35 maps show complex extended structures, with unprecedented detail, that trace the interaction between the molecular gas and the young hot stars. We detect 43 protostars, most of which are situated along the integral-shaped filament extending from the Orion nebula, through OMC 2 and OMC 3. We present high-reliability light curves for some of these objects using the first six epochs of our observing program spread over 6 weeks. We find amplitude variations in excess of 20% for a fraction of the detected protostars over periods as short as a few weeks. This is inconsistent with the dynamical timescales of cool far-IR emitting material that orbits at hundreds of AU from the protostar, and it suggests that the mechanism(s) responsible for the observed variability originates from the inner region of the protostars, likely driven by variable mass accretion.

Journal of Low Temperature Physics, 2013

NIKA (New IRAM KID Arrays) is a dual-band imaging instrument installed at the IRAM (Institut de R... more NIKA (New IRAM KID Arrays) is a dual-band imaging instrument installed at the IRAM (Institut de RadioAstronomie Millimetrique) 30-meter telescope at Pico Veleta (Spain). Two distinct Kinetic Inductance Detectors (KID) focal planes allow the camera to simultaneaously image a field-of-view of about 2 arc-min in the bands 125 to 175 GHz (150 GHz) and 200 to 280 GHz (240 GHz). The sensitivity and stability achieved during the last commissioning Run in June 2013 allows opening the instrument to general observers. We report here the latest results, in particular in terms of sensitivity, now comparable to the state-of-theart Transition Edge Sensors (TES) bolometers, relative and absolute photometry. We describe briefly the next generation NIKA-2 instrument, selected by IRAM to occupy, from 2015, the continuum imager/polarimeter slot at the 30-m telescope.

Astronomy and Astrophysics, 2010

Context. It has been shown that by means of different physical mechanisms the expansion of H ii r... more Context. It has been shown that by means of different physical mechanisms the expansion of H ii regions can trigger the formation of new stars of all masses. This process may be important to the formation of massive stars but has never been quantified in the Galaxy. Aims. We use Herschel-PACS and -SPIRE images from the Herschel Infrared survey of the Galactic plane, Hi-GAL, to perform this study. Methods. We combine the Spitzer-GLIMPSE and -MIPSGAL, radio-continuum and sub-millimeter surveys such as ATLASGAL with Hi-GAL to study Young Stellar Objects (YSOs) observed towards Galactic H ii regions. We select a representative H ii region, N49, located in the field centered on l=30 • observed as part of the Hi-GAL Science Demonstration Phase, to demonstrate the importance Hi-GAL will have to this field of research.

Astronomy and Astrophysics, 2010

The Photodetector Array Camera and Spectrometer (PACS) is one of the three science instruments on... more The Photodetector Array Camera and Spectrometer (PACS) is one of the three science instruments on ESA's far infrared and submillimetre observatory. It employs two Ge:Ga photoconductor arrays (stressed and unstressed) with 16 × 25 pixels, each, and two filled silicon bolometer arrays with 16 × 32 and 32 × 64 pixels, respectively, to perform integral-field spectroscopy and imaging photometry in the 60-210 µm wavelength regime. In photometry mode, it simultaneously images two bands, 60-85 µm or 85-125 µm and 125-210 µm, over a field of view of ∼ 1.75 × 3.5 , with close to Nyquist beam sampling in each band. In spectroscopy mode, it images a field of 47 × 47 , resolved into 5 × 5 pixels, with an instantaneous spectral coverage of ∼ 1500 km/s and a spectral resolution of ∼ 175 km/s. We summarise the design of the instrument, describe observing modes, calibration, and data analysis methods, and present our current assessment of the in-orbit performance of the instrument based on the Performance Verification tests. PACS is fully operational, and the achieved performance is close to or better than the pre-launch predictions.

Astronomy and Astrophysics, 2010

Abstract The Photodetector Array Camera and Spectrometer (PACS) is one of the three science instr... more Abstract The Photodetector Array Camera and Spectrometer (PACS) is one of the three science instruments on ESA's far infrared and submillimetre observatory. It employs two Ge:Ga photoconductor arrays (stressed and unstressed) with pixels, each, and two filled silicon bolometer arrays ...