arXiv subjects

A. Abergel

Publications and source records attributed to A. Abergel.

71 records · Page 4Linked to original sources

arXiv2010

Star formation triggered by the Galactic HII region RCW 120: First results from the Herschel Space Observatory

By means of different physical mechanisms, the expansion of HII regions can promote the formation of new stars of all masses. RCW 120 is a nearby Galactic HII region where triggered star formation occurs. This region is well-studied - there being a wealth of existing data - and is nearby. However, it is surrounded by dense regions for which far infrared data is essential to obtain an unbiased view of the star formation process and in particular to establish whether very young protostars are present. We attempt to identify all Young Stellar Objects (YSOs), especially those previously undetected at shorter wavelengths, to derive their physical properties and obtain insight into the star formation history in this region. We use Herschel-PACS and -SPIRE images to determine the distribution of YSOs observed in the field. We use a spectral energy distribution fitting tool to derive the YSOs physical properties. Herschel-PACS and -SPIRE images confirm the existence of a young source and allow us to determine its nature as a high-mass (8-10 MSun) Class 0 object (whose emission is dominated by a massive envelope) towards the massive condensation 1 observed at (sub)-millimeter wavelengths. This source was not detected at 24 micron and only barely seen in the MISPGAL 70 micron data. Several other red sources are detected at Herschel wavelengths and coincide with the peaks of the millimeter condensations. SED fitting results for the brightest Herschel sources indicate that, apart from the massive Class 0 that forms in condensation 1, young low mass stars are forming around RCW 120. The YSOs observed on the borders of RCW 120 are younger than its ionizing star, which has an age of about 2.5 Myr.

astro-ph.GA↗

arXiv2010

The physical properties of the dust in the RCW 120 HII region as seen by Herschel

Context. RCW 120 is a well-studied, nearby Galactic HII region with ongoing star formation in its surroundings. Previous work has shown that it displays a bubble morphology at mid-infrared wavelengths and has a massive layer of collected neutral material seen at sub-mm wavelengths. Given the well-defined photo-dissociation region (PDR) boundary and collected layer, it is an excellent laboratory to study the "collect and collapse" process of triggered star formation. Using Herschel Space Observatory data at 100, 160, 250, 350, and 500 micron, in combination with Spitzer and APEX-LABOCA data, we can for the first time map the entire spectral energy distribution of an HII region at high angular resolution. Aims. We seek a better understanding of RCW120 and its local environment by analysing its dust temperature distribution. Additionally, we wish to understand how the dust emissivity index, beta, is related to the dust temperature. Methods. We determine dust temperatures in selected regions of the RCW 120 field by fitting their spectral energy distribution (SED), derived using aperture photometry. Additionally, we fit the SED extracted from a grid of positions to create a temperature map. Results. We find a gradient in dust temperature, ranging from >30 K in the interior of RCW 120, to ~20K for the material collected in the PDR, to ~10K toward local infrared dark clouds and cold filaments. Our results suggest that RCW 120 is in the process of destroying the PDR delineating its bubble morphology. The leaked radiation from its interior may influence the creation of the next generation of stars. We find support for an anti-correlation between the fitted temperature and beta, in rough agreement with what has been found previously. The extended wavelength coverage of the Herschel data greatly increases the reliability of this result.

astro-ph.GA↗

arXiv2009

Data inversion for over-resolved spectral imaging in astronomy

We present an original method for reconstructing a three-dimensional object having two spatial dimensions and one spectral dimension from data provided by the infrared slit spectrograph on board the Spitzer Space Telescope. During acquisition, the light flux is deformed by a complex process comprising four main elements (the telescope aperture, the slit, the diffraction grating and optical distortion) before it reaches the two-dimensional sensor. The originality of this work lies in the physical modelling, in integral form, of this process of data formation in continuous variables. The inversion is lso approached with continuous variables in a semi-parametric format decomposing the object into a family of Gaussian functions. The estimate is built in a deterministic regularization framework as the minimizer of a quadratic criterion. These specificities give our method the power to over-resolve. Its performance is illustrated using real and simulated data. We also present a study of the resolution showing a 1.5-fold improvement relative to conventional methods.

astro-ph.IM↗

arXiv2008

Dust processing in photodissociation regions - Mid-IR emission modelling

Mid-infrared spectroscopy of dense illuminated ridges (or photodissociation regions, PDRs) suggests dust evolution. Such evolution must be reflected in the gas physical properties through processes like photo-electric heating or H_2 formation. With Spitzer Infrared Spectrograph (IRS) and ISOCAM data, we study the mid-IR emission of closeby, well known PDRs. Focusing on the band and continuum dust emissions, we follow their relative contributions and analyze their variations in terms of abundance of dust populations. In order to disentangle dust evolution and excitation effects, we use a dust emission model that we couple to radiative transfer. Our dust model reproduces extinction and emission of the standard interstellar medium that we represent with diffuse high galactic latitude clouds called Cirrus. We take the properties of dust in Cirrus as a reference to which we compare the dust emission from more excited regions, namely the Horsehead and the reflection nebula NGC 2023 North. We show that in both regions, radiative transfer effects cannot account for the observed spectral variations. We interpret these variations in term of changes of the relative abundance between polycyclic aromatic hydrocarbons (PAHs, mid-IR band carriers) and very small grains (VSGs, mid-IR continuum carriers). We conclude that the PAH/VSG abundance ratio is 2.4 times smaller at the peak emission of the Horsehead nebula than in the Cirrus case. For NGC2023 North where spectral evolution is observed across the northern PDR, we conclude that this ratio is ~5 times lower in the dense, cold zones of the PDR than in its diffuse illuminated part where dust properties seem to be the same as in Cirrus. We conclude that dust in PDRs seems to evolve from "dense" to "diffuse" properties at the small spatial scale of the dense illuminated ridge.

astro-ph↗

arXiv2007

Aromatic emission from the ionised mane of the Horsehead nebula

We study the evolution of the Aromatic Infrared Bands (AIBs) emitters across the illuminated edge of the Horsehead nebula and especially their survival and properties in the HII region. We present spectral mapping observations taken with the Infrared Spectrograph (IRS) at wavelengths 5.2-38 microns. A strong AIB at 11.3 microns is detected in the HII region, relative to the other AIBs at 6.2, 7.7 and 8.6 microns. The intensity of this band appears to be correlated with the intensity of the [NeII] at 12.8 microns and of Halpha, which shows that the emitters of the 11.3 microns band are located in the ionised gas. The survival of PAHs in the HII region could be due to the moderate intensity of the radiation field (G0 about 100) and the lack of photons with energy above about 25eV. The enhancement of the intensity of the 11.3 microns band in the HII region, relative to the other AIBs can be explained by the presence of neutral PAHs. Our observations highlight a transition region between ionised and neutral PAHs observed with ideal conditions in our Galaxy. A scenario where PAHs can survive in HII regions and be significantly neutral could explain the detection of a prominent 11.3 microns band in other Spitzer observations.

astro-ph↗

arXiv2007

Analysis of the emission of very small dust particles from Spitzer spectro-imagery data using blind signal separation methods

This work was conducted as part of the SPECPDR program, dedicated to the study of very small particles and astrochemistry, in Photo-Dissociation Regions (PDRs). We present the analysis of the mid-IR spectro-imagery observations of Ced 201, NCG 7023 East and North-West and rho-Ophiuchi West filament. Using the data from all four modules of the InfraRed Spectrograph onboard the Spitzer Space Telescope, we produced a spectral cube ranging from 5 to 35 um for each one of the observed PDRs. The resulting cubes were analysed using Blind Signal Separation methods (NMF and FastICA). For Ced 201, rho-Ophiuchi West filament and NGC 7023 East, we find that two signals can be extracted from the original data cubes, which are 5 to 35 um spectra. The main features of the first spectrum are a strong continuum emission at long wavelengths, and a broad 7.8 um band. On the contrary, the second spectrum exhibits the classical Aromatic Infrared Bands (AIBs) and no continuum. The reconstructed spatial distribution maps show that the latter spectrum is mainly present at the cloud surface, close to the star whereas the first one is located slightly deeper inside the PDR. The study of the spectral energy distribution of Ced 201 up to 100 um suggests that, in cool PDRs, the 5-25 um continuum is carried by Very Small Grains (VSGs). The AIB spectra in the observed objects can be interpreted as the contribution of neutral and positively-charged Polycyclic Aromatic Hydrocarbons (PAHs). We extracted the 5 to 25 um emission spectrum of VSGs in cool PDRs, these grains being most likely carbonaceous. We show that the variations of the mid-IR (5-35 um) spectra of PDRs can be explained by the photo-chemical processing of these VSGs and PAHs, VSGs being the progenitors of free PAH.

astro-ph↗

arXiv2005

Density structure of the Horsehead nebula photo-dissociation region

We present high angular resolution images of the H$_2$ 1-0 S(1) line emission obtained with the Son of ISAAC (SOFI) at the New Technology Telescope (NTT) of the Horsehead nebula. These observations are analysed in combination with H$α$ line emission, aromatic dust, CO and dust continuum emissions. The Horsehead nebula illuminated by the O9.5V star $σ$ Ori ($χ\sim$ 60) presents a typical photodissociation region (PDR) viewed nearly edge-on and offers an ideal opportunity to study the gas density structure of a PDR. The H$_2$ fluorescent emission observations reveal extremely sharp and bright filaments associated with the illuminated edge of the nebula which spatially coincides with the aromatic dust emission. Analysis of the H$_2$ fluorescent emission, sensitive to both the far-UV radiation field and the gas density, in conjunction with the aromatic dust and H$α$ line emission, brings new constraints on the illumination conditions and the gas density in the outer PDR region. Furthermore, combination of this data with millimeter observations of CO and dust continuum emission allows us to trace the penetration of the far-UV radiation field into the cloud and probe the gas density structure throughout the PDR. From comparison with PDR model calculations, we find that i) the gas density follows a steep gradient at the cloud edge, with a scale length of 0.02 pc (or 10'') and $n_H\sim 10^4$ and $10^5$ cm$^{-3}$ in the H$_2$ emitting and inner cold molecular layers respectively, and ii) this density gradient model is essentially a constant pressure model, with $P\sim$4 $10^6$ K cm$^{-3}$. The constraints derived here on the gas density profile are important for the study of physical and chemical processes in PDRs and provide new insight into the evolution of interstellar clouds.

astro-ph↗

arXiv2005

Are PAHs precursors of small hydrocarbons in Photo--Dissociation Regions? The Horsehead case

We present maps at high spatial and spectral resolution in emission lines of C2H, c-C3H2, C4H, 12CO and C18O of the edge of the Horsehead nebula obtained with the Plateau de Bure Interferometer (PdBI). The edge of the Horsehead nebula is a one-dimensional Photo--Dissociation Region (PDR) viewed almost edge-on. All hydrocarbons are detected at high signal--to--noise ratio in the PDR where intense emission is seen both in the H2 ro-vibrational lines and in the PAH mid--infrared bands. C18O peaks farther away from the cloud edge. Our observations demonstrate that C2H, cC3H2 and C4H are present in UV--irradiated molecular gas, with abundances nearly as high as in dense, well shielded molecular cores. PDR models i) need a large density gradient at the PDR edge to correctly reproduce the offset between the hydrocarbons and H2 peaks and ii) fail to reproduce the hydrocarbon abundances. We propose that a new formation path of carbon chains, in addition to gas phase chemistry, should be considered in PDRs: because of intense UV--irradiation, large aromatic molecules and small carbon grains may fragment and feed the interstellar medium with small carbon clusters and molecules in significant amount.

astro-ph↗

arXiv2004

ISOCAM observations of the L1551 star formation region

The results of a deep mid-IR ISOCAM survey of the L1551 dark molecular cloud are presented. The aim of this survey is a search for new YSO (Young Stellar Object) candidates, using two broad-band filters centred at 6.7 and 14.3 micron. Although two regions close to the centre of L1551 had to be avoided due to saturation problems, 96 sources were detected in total (76 sources at 6.7 micron and 44 sources at 14.3 micron). Using the 24 sources detected in both filters, 14 were found to have intrinsic mid-IR excess at 14.3 micron and were therefore classified as YSO candidates. Using additional observations in B, V, I, J, H and K obtained from the ground, most candidates detected at these wavelengths were confirmed to have mid-IR excess at 6.7 micron as well, and three additional YSO candidates were found. Prior to this survey only three YSOs were known in the observed region (avoiding L1551 IRS5/NE and HL/XZ Tau). This survey reveals 15 new YSO candidates, although several of these are uncertain due to their extended nature either in the mid-IR or in the optical/near-IR observations. Two of the sources with mid-IR excess are previously known YSOs, one is a brown dwarf MHO 5 and the other is the well known T Tauri star HH30, consisting of an outflow and an optically thick disk seen edge on.

astro-ph↗

arXiv2004

The young stellar population in the Serpens Cloud Core: An ISOCAM survey

We present results from an ISOCAM survey in the two broad band filters LW2 (5-8.5 mu) and LW3 (12-18 mu) of a 0.13 square degree coverage of the Serpens Main Cloud Core. A total of 392 sources were detected in the 6.7 mu band and 139 in the 14.3 mu band to a limiting sensitivity of ~ 2 mJy. Only about 50% of the mid-IR excess sources show excesses in the near-IR J-H/H-K diagram. In the central Cloud Core the Class I/Class II number ratio is 19/18, i.e. about 10 times larger than in other young embedded clusters such as rho Ophiuchi or Chamaeleon. The mid-IR fluxes of the Class I and flat-spectrum sources are found to be on the average larger than those of Class II sources. Stellar luminosities are estimated for the Class II sample, and its luminosity function is compatible with a coeval population of about 2 Myr which follows a three segment power-law IMF. For this age about 20% of the Class IIs are found to be young brown dwarf candidates. The YSOs are in general strongly clustered, the Class I sources more than the Class II sources, and there is an indication of sub-clustering. The sub-clustering of the protostar candidates has a spatial scale of 0.12 pc. These sub-clusters are found along the NW-SE oriented ridge and in very good agreement with the location of dense cores traced by millimeter data. The smallest clustering scale for the Class II sources is about 0.25 pc, similar to what was found for rho Ophiuchi. Our data show evidence that star formation in Serpens has proceeded in several phases, and that a ``microburst'' of star formation has taken place very recently, probably within the last 10^5 yrs.

astro-ph↗

arXiv2004

Carbon budget and carbon chemistry in Photon Dominated Regions

We present a study of small carbon chains and rings in Photon Dominated Regions (PDRs) performed at millimetre wavelengths. Our sample consists of the Horsehead nebula (B33), the rho,Oph L1688 cloud interface, and the cometary-shaped cloud IC63. Using the IRAM 30-m telescope, the SEST and the Effelsberg 100-m teles cope at Effelsberg., we mapped the emission of \cch, c-C3H2 and C4H, and searched for heavy hydrocarbons such as c-C3H, l-C3H, l-C3H2, l-C4H2 and C6H. The large scale maps show that small hydrocarbons are present until the edge of all PDRs, which is surprising as they are expected to be easily destroyed by UV radiation. Their spatial distribution reasonably agrees with the aromatic emission mapped in mid-IR wavelength bands. Their abundances relative to H2 are relatively high and comparable to the ones derived in dark clouds such as L134N or TMC-1, known as efficient carbon factories. In particular, we report the first detection of C6H in a PDR. We have run steady-state PDR models using several gas-phase chemical networks (UMIST95 and the New Standard Model) and conclude that both networks fail in reproducing the high abundances of some of these hydrocarbons by an order of magnitude. The high abundance of hydrocarbons in the PDR may suggest that the photo-erosion of UV-irradiated large carbonaceous compounds could efficiently feed the ISM with small carbon clusters or molecules. This new production mechanism of carbon chains and rings could overcome their destruction by the UV radiation field. Dedicated theoretical and laboratory measurements are required in order to understand and implement these additional chemical routes.

astro-ph↗

arXiv2001

ISOCAM observations of the rho Ophiuchi cloud: Luminosity and mass functions of the pre-main sequence embedded cluster

We present the results of the first extensive mid-infrared (IR) imaging survey of the rho Ophiuchi embedded cluster, performed with the ISOCAM camera on board the ISO satellite. The main molecular cloud L1688, as well as L1689N and L1689S, have been completely surveyed for point sources at 6.7 and 14.3 micron. A total of 425 sources are detected including 16 Class I, 123 Class II, and 77 Class III young stellar objects (YSOs). Essentially all of the mid-IR sources coincide with near-IR sources, but a large proportion of them are recognized for the first time as YSOs. Our dual-wavelength survey allows us to identify essentially all the YSOs with IR excess in the embedded cluster down to Fnu ~ 10 - 15 mJy. It more than doubles the known population of Class II YSOs and represents the most complete census to date of newly formed stars in the rho Ophiuchi central region. The stellar luminosity function of the complete sample of Class II YSOs is derived with a good accuracy down to L= 0.03 Lsun. A modeling of this lumino- sity function, using available pre-main sequence tracks and plausible star for- mation histories, allows us to derive the mass distribution of the Class II YSOs which arguably reflects the IMF of the embedded cluster. We estimate that the IMF in rho Ophiuchi is well described by a two-component power law with a low- mass index of -0.35+/-0.25, a high-mass index of -1.7 (to be compared with the Salpeter value of -1.35), and a break occurring at M = 0.55+/-0.25 Msun. This IMF is flat with no evidence for a low-mass cutoff down to at least 0.06 Msun.

astro-ph↗

arXiv2000

Optimizing ISOCAM data processing using spatial redundancy

We present new data processing techniques that allow to correct the main instrumental effects that degrade the images obtained by ISOCAM, the camera on board the Infrared Space Observatory (ISO). Our techniques take advantage of the fact that a position on the sky has been observed by several pixels at different times. We use this information (1) to correct the long term variation of the detector response, (2) to correct memory effects after glitches and point sources, and (3) to refine the deglitching process. Our new method allows the detection of faint extended emission with contrast smaller than 1% of the zodiacal background. The data reduction corrects instrumental effects to the point where the noise in the final map is dominated by the readout and the photon noises. All raster ISOCAM observations can benefit from the data processing described here. These techniques could also be applied to other raster type observations (e.g. ISOPHOT or IRAC on SIRTF).

astro-ph↗

arXiv2000

The extragalactic background and its fluctuations in the far-infrared wavelengths

A Cosmic Far-InfraRed Background (CFIRB) has long been predicted that would traces the intial phases of galaxy formation. It has been first detected by Puget et al.(1996) using COBE data and has been later confirmed by several recent studies (Fixsen et al. 1998, Hauser et al. 1998, Lagache et al. 1999). We will present a new determination of the CFIRB that uses for the first time, in addition to COBE data, two independent gas tracers: the HI survey of Leiden/Dwingeloo (hartmann, 1998) and the WHAM H$_α$ survey (Reynolds et al 1998). We will see that the CFIRB above 100 micron is now very well constrained. The next step is to see if we can detect its fluctuations. To search for the CFIRB fluctuations, we have used the FIRBACK observations. FIRBACK is a deep cosmological survey conducted at 170 micron with ISOPHOT (Dole et al., 2000). We show that the emission of unresolved extra-galactic sources clearly dominates, at arcminute scales, the background fluctuations in the lowest galactic emission regions. This is the first detection of the CFIRB fluctuations.

astro-ph↗

arXiv1999

First detection of the WIM dust emission. Implication for the Cosmic Far-Infrared Background

We present a new analysis of the far-IR emission at high Galactic latitude based on COBE and HI data. A decomposition of the Far-IR emission over the HI, H^+ and H_2 Galactic gas components and the Cosmic Far InfraRed Background (CFIRB) is described. For the first time the far-IR emission of dust associated with the Warm Ionised Medium (WIM) is evidenced. This component determined on about 25% of the sky is detected at a 10 sigma level in the [200, 350]micron band. The best representation of the WIM dust spectrum is obtained for a temperature of 29.1 K and an emissivity law $tau/N_H+=3.8 \pm 0.8 10^{-26} (λ/250\mic)^{-1} cm^2$. With a spectral index equal to 2, the emissivity law becomes $τ/N_H^+= 1.0 \pm 0.2 10^{-25} (λ/250\mic)^{-2} cm^2$, with a temperature of 20 K, which is significantly higher than the temperature of dust associated with HI gas. The variation in the dust spectrum from the HI to the WIM component can be explained by only changing the upper cutoff of the Big Grain size distribution from 0.1 micron to 30 nm. The detection of IR emission of dust in the WIM significantly decreases the intensity of the CFIRB, especially around 200 micron which corresponds to the peak of energy.

astro-ph↗

arXiv1998

The interstellar cold dust observed by COBE

Using DIRBE and FIRAS maps at high latitude ($|b|>10^{\circ}$) we derive the spatial distribution of the dust temperature associated with the diffuse cirrus and the dense molecular clouds. For a $ν^2$ emissivity law, we find that the equilibrium dust temperature of the cirrus is about 17.5 K with only small variations over the high latitude sky. Comparison of the far Infrared DIRBE maps shows the presence of a colder emission component with a temperature around 15 K, assuming a $ν^2$ emissivity law. The lowest values of the temperature found in the cold regions ($\sim 13 K$) are compatible with the results recently obtained for dense cores in star forming regions by the balloon-borne experiment SPM-PRONAOS (Ristorcelli et al., 1996, 1998, Serra et al., 1997). This cold component is in particular present in the direction of known molecular complexes with low star forming activity such as Taurus. The association between the cold component and molecular clouds is further demonstrated by the fact that all sky pixels with significant cold emission have an excess IR emission with respect to the high latitude IR/HI correlation. We have deduced a threshold value of the column density, $N_{HI} = 2.5 10^{20} H/cm^2$, below which cold dust is not detected within the FIRAS beam of $\sim 7\degr$. We have re-examined the problem of the existence of a very cold dust component ($T \sim 7 K$) by combining DIRBE maps of the cold emission with FIRAS spectra, corrected for the isotropic component found in Puget et al. (1996). The warm and cold component deduced from the analysis of DIRBE maps account for the Galactic FIRAS spectra with no need for a very cold component ($T\sim7$ K).

astro-ph↗

arXiv1998

A classical approach to faint extragalactic source extraction from ISOCAM deep surveys. Application to the Hubble Deep Field

We have developed a general data reduction technique for ISOCAM data coming from various deep surveys of faint galaxies. In order to reach the fundamental limits of the camera (due to the background photon noise and the readout noise), we have devised several steps in the reduction processes that transform the raw data to a sky map which is then used for point source and sligthly extended source extraction. We show illustrative examples of our present scheme by using data taken from the publicly available Hubble Deep Field ISOCAM survey in order to demonstrate its characteristics. More than thirty sources down to the 60 (resp. 100) microJy level are clearly detected above 4 sigma at wavelengths of 7 (resp. 15) micron, the vast majority at 15 micron. A large fraction of these sources can be identified with visible objects of median magnitude 22 and K-band magnitude of 17.5 and redshifts between 0.5 and 1 (when available). A few very red objects could be at larger redshifts.

astro-ph↗