arXiv subjects

F. Massi

Publications and source records attributed to F. Massi.

At least 37 records · Page 2Linked to original sources

arXiv2014

The fiber-fed preslit of GIANO at T.N.G

Giano is a Cryogenic Spectrograph located in T.N.G. (Spain) and commisioned in 2013. It works in the range 950-2500 nm with a resolving power of 50000. This instrument was designed and built for direct feeding from the telescope [2]. However, due to constraints imposed on the telescope interfacing during the pre-commissioning phase, it had to be positioned on the rotating building, far from the telescope focus. Therefore, a new interface to the telescope, based on IR-transmitting ZBLAN fibers with 85μm core, was developed.Originally designed to work directly at the $f/11$ nasmyth focus of the telescope, in 2011 it has decided to use a fiber to feed it. The beam from the telescope is focused on a double fiber boundle by a Preslit Optical Bench attached to the Nasmith A interface of the telescope. This Optical Bench contains the fiber feeding system and other important features as a guiding system, a fiber viewer, a fiber feed calibration lamp and a nodding facility between the two fibers. The use of two fibers allow us to have in the echellogram two spectrograms side by side in the same acquisition: one of the star and the other of the sky or simultaneously to have the star and a calibration lamp. Before entering the cryostat the light from the fiber is collectd by a second Preslit Optical Bench attached directly to the Giano cryostat: on this bench the correct f-number to illuminate the cold stop is generated and on the same bench is placed an image slicer to increase the efficiency of the system.

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arXiv2014

Trade-off study for high resolution spectroscopy in the near infrared with ELT telescopes: seeing-limited vs. diffraction limited instruments

HIRES, a high resolution spectrometer, is one of the first five instruments foreseen in the ESO roadmap for the E-ELT. This spectrograph should ideally provide full spectral coverage from the UV limit to 2.5 microns, with a resolving power from R$\sim$10,000 to R$\sim$100,000. At visual/blue wavelengths, where the adaptive optics (AO) cannot provide an efficient light-concentration, HIRES will necessarily be a bulky, seeing-limited instrument. The fundamental question, which we address in this paper, is whether the same approach should be adopted in the near-infrared range, or HIRES should only be equipped with compact infrared module(s) with a much smaller aperture, taking advantage of an AO-correction. The main drawbacks of a seeing-limited instrument at all wavelengths are: \textit{i)} Lower sensitivities at wavelengths dominated by thermal background (red part of the K-band). \textit{ii)} Much higher volumes and costs for the IR spectrograph module(s). The main drawbacks of using smaller, AO-fed IR module(s) are: \textit{i)} Performances rapidly degrading towards shorter wavelengths (especially J e Y bands). \textit{ii)} Different spatial sampling of extended objects (the optical module see a much larger area on the sky). In this paper we perform a trade-off analysis and quantify the various effects that contribute to improve or deteriorate the signal to noise ratio. In particular, we evaluate the position of the cross-over wavelength at which AO-fed instruments starts to outperform seeing-limited instruments. This parameter is of paramount importance for the design of the part of HIRES covering the K-band.

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arXiv2014

The inner circumstellar disk of the UX Ori star V1026 Sco

The UX Ori type variables (named after the prototype of their class) are intermediate-mass pre-main sequence objects. One of the most likely causes of their variability is the obscuration of the central star by orbiting dust clouds. We investigate the structure of the circumstellar environment of the UX~Ori star V1026 Sco (HD 142666) and test whether the disk inclination is large enough to explain the UX Ori variability. We observed the object in the low-resolution mode of the near-infrared interferometric VLTI/AMBER instrument and derived H- and K-band visibilities and closure phases. We modeled our AMBER observations, published Keck Interferometer observations, archival MIDI/VLTI visibilities, and the spectral energy distribution using geometric and temperature-gradient models. Employing a geometric inclined-ring disk model, we find a ring radius of 0.15 +- 0.06 AU in the H band and 0.18 +- 0.06 AU in the K band. The best-fit temperature-gradient model consists of a star and two concentric, ring-shaped disks. The inner disk has a temperature of 1257^{+133}_{-53} K at the inner rim and extends from 0.19 +- 0.01 AU to 0.23 +- 0.02 AU. The outer disk begins at 1.35^{+0.19}_{-0.20} AU and has an inner temperature of 334^{+35}_{-17} K. The derived inclination of 48.6^{+2.9}_{-3.6}deg approximately agrees with the inclination derived with the geometric model (49 +- 5deg in the K band and 50 +- 11deg in the H band). The position angle of the fitted geometric and temperature-gradient models are 163 +- 9deg (K band; 179 +- 17deg in the H band) and 169.3^{+4.2}_{-6.7}deg, respectively. The narrow width of the inner ring-shaped model disk and the disk gap might be an indication for a puffed-up inner rim shadowing outer parts of the disk. The intermediate inclination of ~50deg is consistent with models of UX Ori objects where dust clouds in the inclined disk obscure the central star.

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arXiv2013

GIANO-TNG spectroscopy of red supergiants in the young star cluster RSGC2

The inner disk of the Galaxy has a number of young star clusters dominated by red supergiants that are heavily obscured by dust extinction and observable only at infrared wavelengths. These clusters are important tracers of the recent star formation and chemical enrichment history in the inner Galaxy. During the technical commissioning and as a first science verification of the GIANO spectrograph at the Telescopio Nazionale Galileo, we secured high-resolution (R~50,000) near-infrared spectra of three red supergiants in the young Scutum cluster RSGC2. Taking advantage of the full YJHK spectral coverage of GIANO in a single exposure, we were able to identify several tens of atomic and molecular lines suitable for chemical abundance determinations. By means of spectral synthesis and line equivalent width measurements, we obtained abundances of Fe and other iron-peak elements such as V, Cr, Ni, of alpha (O, Mg, Si, Ca and Ti) and other light elements (C, N, Na, Al, K, Sc), and of some s-process elements (Y, Sr). We found iron abundances between half and one third solar and solar-scaled [X/Fe] abundance patterns of iron-peak, alpha and most of the light elements, consistent with a thin-disk chemistry. We found a depletion of [C/Fe] and enhancement of [N/Fe], consistent with CN burning, and low 12C/13C abundance ratios (between 9 and 11), requiring extra-mixing processes in the stellar interiors during the post-main sequence evolution. Finally, we found a slight [Sr/Fe] enhancement and a slight [Y/Fe] depletion (by a factor of <=2), with respect to solar.

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arXiv2013

A GIANO-TNG high resolution IR spectrum of the airglow emission

A flux-calibrated high resolution spectrum of the airglow emission is a practical lambda-calibration reference for astronomical spectral observations. It is also useful for constraining the molecular parameters of the OH molecule and the physical conditions in the upper mesosphere. methods: We use the data collected during the first technical commissioning of the GIANO spectrograph at the Telescopio Nazionale Galileo (TNG). The high resolution (R~50,000) spectrum simultaneously covers the 0.95-2.4 micron wavelength range. Relative flux calibration is achieved by the simultaneous observation of spectrophotometric standard star. results: We derive a list of improved positions and intensities of OH infrared lines. The list includes Lambda-split doublets many of which are spectrally resolved. Compared to previous works, the new results correct errors in the wavelengths of the Q-branch transitions. The relative fluxes of OH lines from different vibrational bands show remarkable deviations from theoretical predictions: the Deltav=3,4 lines are a factor of 2 and 4 brighter than expected. We also find evidence of a significant fraction (1-4%) of OH molecules with ``non-thermal'' population of high-J levels. Finally we list wavelengths and fluxes of 153 lines not attributable to OH. Most of these can be associated to O2, while 37 lines in the H band are not identified. The O2 and unidentified lines in the H band account for ~5% of the total airglow flux in this band.

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arXiv2013

Spitzer-IRAC survey of molecular jets in Vela-D

We present a survey of H2 jets from young protostars in the Vela-D molecular cloud (VMR-D), based on Spitzer -IRAC data between 3.6 and 8.0 micron. Our search has led to the identification of 15 jets and about 70 well aligned knots within 1.2 squared degree. We compare the IRAC maps with observations of the H2 1-0 S(1) line at 2.12 micron, with a Spitzer-MIPS map at 24 and 70 micron, and with a map of the dust continuum emission at 1.2 mm. We find a association between molecular jets and dust peaks. The jet candidate exciting sources have been searched for in the published catalog of the Young Stellar Objects of VMR-D. We selected all the sources of Class II or earlier which are located close to the jet center and aligned with it.The association between jet and exciting source was validated by estimating the differential extinction between the jet opposite lobes. We are able to find a best-candidate exciting source in all but two jets. Four exciting sources are not (or very barely) observed at wavelengths shorter than 24 micron, suggesting they are very young protostars. Three of them are also associated with the most compact jets. The exciting source Spectral Energy Distributions have been modeled by means of the photometric data between 1.2 micron and 1.2 mm. From SEDs fits we derive the main source parameters, which indicate that most of them are low-mass protostars. A significant correlation is found between the projected jet length and the [24] - [70] color, which is consistent with an evolutionary scenario according to which shorter jets are associated with younger sources. A rough correlation is found between IRAC line cooling and exciting source bolometric luminosity, in agreement with the previous literature. The emerging trend suggests that mass loss and mass accretion are tightly related phenomena and that both decrease with time.

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arXiv2013

Pre-main-sequence binaries with tidally disrupted discs: the Br gamma in HD 104237

Active pre-main-sequence binaries with separations of around ten stellar radii present a wealth of phenomena unobserved in common systems. The study of these objects is extended from Classical T Tauri stars to the Herbig Ae star HD 104237. Spectro-interferometry with the VLTI/AMBER is presented. It is found that the K-band continuum squared visibilities are compatible with a circumbinary disc with a radius of ~0.5 AU. However, a significant fraction (~50 per cent) of the flux is unresolved and not fully accounted by the stellar photospheres. The stars probably don't hold circumstellar discs, in addition to the circumbinary disk, due to the combined effects of inner magnetospheric truncation and outer tidal truncation. This unresolved flux likely arises in compact structures inside the tidally disrupted circumbinary disc. Most ($\gtrsim 90$ per cent) of the Br gamma line emission is unresolved. The line-to-continuum spectro-astrometry shifts in time, along the direction of the Ly alpha jet known to be driven by the system. The shift is anti-correlated with the Br gamma equivalent width. It is shown that the unresolved Br gamma emission cannot originate in the jet but instead is compatible with stellar emission from the orbiting binary components. The increase in the absolute value of the equivalent width of the line takes place at periastron passage; it could arise in an accretion burst, a flare or in the increase in effective size of the emission region by the interaction of the magnetospheres. The binary longitude of the ascending node is found to be $Ω=(235\pm3)\degr$ and the orbit retrograde. The origin of the jet is revisited. The tidal disruption of the circumstellar disks creates difficulties to ejection models that rely on stellar magnetosphere and disc coupling. A scenario of a stellar wind collimated by a circumbinary disc wind is suggested.

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arXiv2013

Evidence of an asymmetrical Keplerian disk in the Brγ and He I emission lines around the Be star HD 110432

Context. HD 110432 was classified as a "γCas X-ray analog" since it has similar peculiar X-ray and optical characteristics, i.e. a hard-thermal X-ray variable emission and an optical spectrum affected by an extensive disk. Lopes de Oliveira et al. (2007) suggest that it might be a Be star harboring an accreting white dwarf or that the X-rays may come from an interaction between the surface of the star and its disk. Aims. To investigate the disk around this Be star we used the VLTI/AMBER instrument, which combines high spectral (R=12000) and high spatial (θmin =4 mas) resolutions. Methods. We constrain the geometry and kinematics of its circumstellar disk from the highest spatial resolution ever achieved on this star. Results. We obtain a disk extension in the Brγ line of 10.2 D\ast and 7.8 D\ast in the He I line at 2.05 μm assuming a Gaussian disk model. The disk is clearly following a Keplerian rotation. We obtained an inclination angle of 55\degree, and the star is a nearly critical rotator with Vrot /Vc =1.00$\pm$0.2. This inclination is greater than the value found for γCas (about 42\degree, Stee et al. 2012), and is consistent with the inference from optical Fe II emission profiles by Smith & Balona (2006) that the inclination should be more than the γCas value. In the near-IR continuum, the disk of HD 110432 is 3 times larger than γCas's disk. We have no direct evidence of a companion around HD 110432, but it seems that we have a clear signature for disk inhomogeneities as detected for ζ Tau. This asymmetrical disk detection may be interpreted within the one-armed oscillation viscous disk framework. Another finding is that the disk size in the near-IR is similar to other Be stars with different spectral types and thus may be independent of the stellar parameters, as found for classical Be stars.

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arXiv2012

AMBER/VLTI observations of the B[e] star MWC 300

Aims. We study the enigmatic B[e] star MWC 300 to investigate its disk and binary with milli-arcsecond-scale angular resolution. Methods. We observed MWC 300 with the VLTI/AMBER instrument in the H and K bands and compared these observations with temperature-gradient models to derive model parameters. Results. The measured low visibility values, wavelength dependence of the visibilities, and wavelength dependence of the closure phase directly suggest that MWC 300 consists of a resolved disk and a close binary. We present a model consisting of a binary and a temperature-gradient disk that is able to reproduce the visibilities, closure phases, and spectral energy distribution. This model allows us to constrain the projected binary separation (~4.4 mas or ~7.9 AU), the flux ratio of the binary components (~2.2), the disk temperature power-law index, and other parameters.

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arXiv2012

Revealing the inner circumstellar disk of the T Tauri star S CrA N using the VLTI

Aims: We investigate the structure of the circumstellar disk of the T Tauri star S CrA N and test whether the observations agree with the standard picture proposed for Herbig Ae stars. Methods: Our observations were carried out with the VLTI/AMBER instrument in the H and K bands with the low spectral resolution mode. For the interpretation of our near-infrared AMBER and archival mid-infrared MIDI visibilities, we employed both geometric and temperature-gradient models. Results: To characterize the disk size, we first fitted geometric models consisting of a stellar point source, a ring-shaped disk, and a halo structure to the visibilities. In the H and K bands, we measured ring-fit radii of 0.73 +- 0.03 mas (corresponding to 0.095 +- 0.018 AU for a distance of 130 pc) and 0.85 +- 0.07 mas (0.111 +- 0.026 AU), respectively. This K-band radius is approximately two times larger than the dust sublimation radius of ~0.05 AU expected for a dust sublimation temperature of 1500 K and gray dust opacities, but approximately agrees with the prediction of models including backwarming (namely a radius of ~0.12 AU). The derived temperature-gradient models suggest that the disk is approximately face-on consisting of two disk components with a gap between star and disk. The inner disk component has a temperature close to the dust sublimation temperature and a quite narrow intensity distribution with a radial extension from 0.11 AU to 0.14 AU. Conclusions: Both our geometric and temperature-gradient models suggest that the T Tauri star S CrA N is surrounded by a circumstellar disk that is truncated at an inner radius of ~0.11 AU. The narrow extension of the inner temperature-gradient disk component implies that there is a hot inner rim.

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arXiv2012

Close encounters of the protostellar kind in IC 1396N

We have mapped in the 2.7 mm continuum and 12CO with the PdBI the IR-dark "tail" that crosses the IC 1396N globule from south to north, and is the most extincted part of this cloud. These observations have allowed us to distinguish all possible associations of molecular hydrogen emission features by revealing the presence of two well-collimated low-mass protostellar outflows at the northern part of the globule. The outflows are located almost in the plane of the sky and are colliding with each other towards the position of a strong 2.12 microns H2 line emission feature.

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arXiv2011

Spatially resolving the outer atmosphere of the M giant BK Vir in the CO first overtone lines with VLTI/AMBER

The mass-loss mechanism in normal K--M giant stars with small variability amplitudes is not yet understood, although they are the majority among red giant stars. We present high-spatial and high-spectral resolution observations of the 2.3 micron CO lines in the M7 giant BK Vir with a spatial resolution of 9.8 mas and a spectral resolution of 12000, using AMBER at the Very Large Telescope Interferometer (VLTI). The angular diameters observed in the CO lines are 12--31% larger than those measured in the continuum. We also detected asymmetry in the CO line-forming region. The data taken 1.5 months apart show possible time variation on a spatial scale of 30 mas (corresponding to 3 x stellar diameter) at the CO band head. Comparison of the observed data with the MARCS photospheric model shows that whereas the observed CO line spectrum can be well reproduced by the model, the angular sizes observed in the CO lines are much larger than predicted by the model. Our model with two additional CO layers above the MARCS photosphere reproduces the observed spectrum and interferometric data in the CO lines simultaneously. This model suggests that the inner CO layer at ~1.2 stellar radii is very dense and warm with a CO column density of ~10^{22} cm^{-2} and temperatures of 1900--2100K, while the outer CO layer at 2.5--3.0 stellar radii is characterized by column densities of 10^{19}--10^{20} cm^{-2} and temperatures of 1500--2100K. Our AMBER observations of BK Vir have spatially resolved the extended molecular outer atmosphere of a normal M giant in the individual CO lines for the first time. The temperatures derived for the CO layers are higher than or equal to the uppermost layer of the MARCS photospheric model, implying the operation of some heating mechanism in the outer atmosphere.

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arXiv2011

Molecular clouds under the influence of massive stars in the Galactic HII region G353.2+0.9

The Galactic HII region G353.2+0.9 is excited by the massive open cluster Pismis-24. By analyzing (sub-)mm molecular-line and -continuum we study the detailed morphology of the gas and dust, as well as their physical parameters and their variation across the PDR. We observed various molecules and transitions to derive the physical properties of the molecular gas through line ratios, and both LTE and non-LTE analyses. The physical properties of the gas were derived with a Bayesian approach for the non-LTE analysis. Based on the continuum data at 870 micron, we derived the dust mass and the column density of H2, and thus the molecular abundances. The total mass of the gas in the region is ca. 2000 Mo, while that of the dust is ca. 21 Mo. A velocity gradient in the region suggests that the expansion of the ionized gas is pushing the molecular gas away from the observer. We unambiguously identify the ionization front, at the location of which we detect an increase in gas density and temperature. We find at least 14 clumps at different positions and LSR velocities. We derive kinetic temperatures in the ranges 11-45 K (CS) and 20-45 K (CN). The H2 number density is typically around 1e5 cm^-3 from CS and few 1e5 cm^-3 from CN, with maxima above 1e6 cm^-3. The abundances of the molecules observed vary across the region, and appear to be higher in regions further away from the ionization front.

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arXiv2011

A low optical depth region in the inner disk of the HerbigAe star HR5999

Circumstellar disks surrounding young stars are known to be the birthplaces of planets, and the innermost astronomical unit is of particular interest. We present new long-baseline spectro-interferometric observations of the HerbigAe star, HR5999, obtained in the H and K bands with the AMBER instrument at the VLTI, and aim to produce near-infrared images at the sub-AU spatial scale. We spatially resolve the circumstellar material and reconstruct images using the MiRA algorithm. In addition, we interpret the interferometric observations using models that assume that the near-infrared excess is dominated by the emission of a circumstellar disk. We compare the images reconstructed from the VLTI measurements to images obtained using simulated model data. The K-band image reveals three main elements: a ring-like feature located at ~0.65 AU, a low surface brightness region inside, and a central spot. At the maximum angular resolution of our observations (1.3 mas), the ring is resolved while the central spot is only marginally resolved, preventing us from revealing the exact morphology of the circumstellar environment. We suggest that the ring traces silicate condensation, i.e., an opacity change, in a circumstellar disk around HR 5999. We build a model that includes a ring at the silicate sublimation radius and an inner disk of low surface brightness responsible for a large amount of the near-infrared continuum emission. The model successfully fits the SED, visibilities, and closure phases, and provides evidence of a low surface brightness region inside the silicate sublimation radius. This study provides additional evidence that in HerbigAe stars, there is material in a low surface brightness region, probably a low optical depth region, located inside the silicate sublimation radius and of unknown nature.

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arXiv2011

Constraining the structure of the planet-forming region in the disk of the Herbig Be star HD 100546

Studying the physical conditions in circumstellar disks is a crucial step toward understanding planet formation. Of particular interest is the case of HD 100546, a Herbig Be star that presents a gap within the first 13 AU of its protoplanetary disk, that may originate in the dynamical interactions of a forming planet. We gathered a large amount of new interferometric data using the AMBER/VLTI instrument in the H- and K-bands to spatially resolve the warm inner disk and constrain its structure. Then, combining these measurements with photometric observations, we analyze the circumstellar environment of HD 100546 in the light of a passive disk model based on 3D Monte-Carlo radiative transfer. Finally, we use hydrodynamical simulations of gap formation by planets to predict the radial surface density profile of the disk and test the hypothesis of ongoing planet formation. The SED and the NIR interferometric data are adequately reproduced by our model. We show that the H- and K-band emissions are coming mostly from the inner edge of the internal dust disk, located near 0.24 AU from the star, i.e., at the dust sublimation radius in our model. We directly measure an inclination of $33^{\circ} \pm 11^{\circ}$ and a position angle of $140^{\circ} \pm 16^{\circ}$ for the inner disk. This is similar to the values found for the outer disk ($i \simeq 42^{\circ}$, $PA \simeq 145^{\circ}$), suggesting that both disks may be coplanar. We finally show that 1 to 8 Jupiter mass planets located at $\sim 8$ AU from the star would have enough time to create the gap and the required surface density jump of three orders of magnitude between the inner and outer disk. However, no information on the amount of matter left in the gap is available, which precludes us from setting precise limits on the planet mass, for now.

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arXiv2010

The outburst of an embedded low-mass YSO in L1641

Strong outbursts in very young and embedded protostars are rare and not yet fully understood. They are believed to originate from an increase of the mass accretion rate onto the source. We report the discovery of a strong outburst in a low-mass embedded young stellar object (YSO), namely 2MASS-J05424848-0816347 or [CTF93]216-2, as well as its photometric and spectroscopic follow-up. Using near- to mid-IR photometry and NIR low-resolution spectroscopy, we monitor the outburst, deriving its magnitude, duration, as well as the enhanced accretion luminosity and mass accretion rate. [CTF93]216-2 increased in brightness by ~4.6, 4.0, 3.8, and 1.9 mag in the J, H, Ks bands and at 24 um, respectively, corresponding to an L_bol increase of ~20 L_sun. Its early spectrum, probably taken soon after the outburst, displays a steep almost featureless continuum, with strong CO band heads and H_2O broad-band absorption features, and Br gamma line in emission. A later spectrum reveals more absorption features, allowing us to estimate T_eff~3200 K, M~0.25 M_sun, and mass accretion rate~1.2x10^{-6} M_sun yr^{-1}. This makes it one of the lowest mass YSOs with a strong outburst so far discovered.

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arXiv2010

The 2008 outburst in the young stellar system ZCMa: I. Evidence of an enhanced bipolar wind on the AU-scale

Accretion is a fundamental process in star formation. Although the time evolution of accretion remains a matter of debate, observations and modelling studies suggest that episodic outbursts of strong accretion may dominate the formation of the protostar. Observing young stellar objects during these elevated accretion states is crucial to understanding the origin of unsteady accretion. ZCMa is a pre-main-sequence binary system composed of an embedded Herbig Be star, undergoing photometric outbursts, and a FU Orionis star. The Herbig Be component recently underwent its largest optical photometric outburst detected so far. We aim to constrain the origin of this outburst by studying the emission region of the HI Brackett gamma line, a powerful tracer of accretion/ejection processes on the AU-scale in young stars. Using the AMBER/VLTI instrument at spectral resolutions of 1500 and 12 000, we performed spatially and spectrally resolved interferometric observations of the hot gas emitting across the Brackett gamma emission line, during and after the outburst. From the visibilities and differential phases, we derive characteristic sizes for the Brackett gamma emission and spectro-astrometric measurements across the line, with respect to the continuum. We find that the line profile, the astrometric signal, and the visibilities are inconsistent with the signature of either a Keplerian disk or infall of matter. They are, instead, evidence of a bipolar wind, maybe partly seen through a disk hole inside the dust sublimation radius. The disappearance of the Brackett gamma emission line after the outburst suggests that the outburst is related to a period of strong mass loss rather than a change of the extinction along the line of sight. Based on these conclusions, we speculate that the origin of the outburst is an event of enhanced mass accretion, similar to those occuring in EX Ors and FU Ors.

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arXiv2010

The Spitzer-IRAC Point Source Catalog of the Vela-D Cloud

This paper presents the observations of the Cloud D in the Vela Molecular Ridge, obtained with the IRAC camera onboard the Spitzer Space Telescope at the wavelengths λ= 3.6, 4.5, 5.8, 8.0 μm. A photometric catalog of point sources, covering a field of approximately 1.2 square degrees, has been extracted and complemented with additional available observational data in the millimeter region. Previous observations of the same region, obtained with the Spitzer MIPS camera in the photometric bands at 24 μm and 70 μm, have also been reconsidered to allow an estimate of the spectral slope of the sources in a wider spectral range. A total of 170,299 point sources, detected at the 5-sigma sensitivity level in at least one of the IRAC bands, have been reported in the catalog. There were 8796 sources for which good quality photometry was obtained in all four IRAC bands. For this sample, a preliminary characterization of the young stellar population based on the determination of spectral slope is discussed; combining this with diagnostics in the color-magnitude and color-color diagrams, the relative population of young stellar objects in the different evolutionary classes has been estimated and a total of 637 candidate YSOs have been selected. The main differences in their relative abundances have been highlighted and a brief account for their spatial distribution is given. The star formation rate has been also estimated and compared with the values derived for other star forming regions. Finally, an analysis of the spatial distribution of the sources by means of the two-point correlation function shows that the younger population, constituted by the Class I and flat-spectrum sources, is significantly more clustered than the Class II and III sources.

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