arXiv subjects

M. Robberto

Publications and source records attributed to M. Robberto.

At least 37 records · Page 2Linked to original sources

arXiv2011

The low-mass Initial Mass Function in the Orion Nebula cluster based on HST/NICMOS III imaging

We present deep HST/NICMOS Camera 3 F110W and F160W imaging of a 26'x33', corresponding to 3.1pcx3.8pc, non-contiguous field towards the Orion Nebula Cluster (ONC). The main aim is to determine the ratio of low--mass stars to brown dwarfs for the cluster as a function of radius out to a radial distance of 1.5pc. The sensitivity of the data outside the nebulous central region is F160W=21.0 mag, significantly deeper than previous studies of the region over a comparable area. We create an extinction limited sample and determine the ratio of low-mass stars (0.08-1Msun) to brown dwarfs (0.02-0.08Msun and 0.03-0.08Msun) for the cluster as a whole and for several annuli. The ratio found for the cluster within a radius of 1.5pc is R(02)=N(0.08-1Msun)/N(0.02-0.08Msun)=1.7+-0.2, and R(03)=N(0.08-1Msun)/N(0.03-0.08Msun)=2.4+-0.2, after correcting for field stars. The ratio for the central 0.3pcx0.3pc region down to 0.03Msun was previously found to be R(03)=3.3+0.8-0.7, suggesting the low-mass content of the cluster is mass segregated. We discuss the implications of a gradient in the ratio of stars to brown dwarfs in the ONC in the context of previous measurements of the cluster and for other nearby star forming regions. We further discuss the current evidence for variations in the low-mass IMF and primordial mass segregation.

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arXiv2011

Quantitative Evidence for an Intrinsic Age Spread in the Orion Nebula Cluster

Aims. We present a study of the distribution of stellar ages in the Orion Nebula Cluster (ONC) based on accurate HST photometry taken from the HST Treasury Program observations of the ONC utilizing the most recent estimate of the cluster's distance (Menten et al. 2007). We investigate the presence of an intrinsic age spread in the region and a possible trend of age with the spatial distribution. Methods. We estimate the extinction and accretion luminosity towards each source by performing synthetic photometry on an empirical calibration of atmospheric models (Da Rio et al. 2010) using the package Chorizos (Maiz-Apellaniz 2004). The position of the sources in the HR-diagram is compared with different theoretical isochrones to estimate the mean cluster age and age dispersion. Through Monte Carlo simulations we quantify the amount of intrinsic age spread in the region, taking into account uncertainties on the distance, spectral type, extinction, unresolved binaries, accretion and photometric variability. Results. According to Siess et al. (2000) evolutionary models the mean age of the Cluster is 2.2 Myr with a scatter of few Myrs. With Monte Carlo simulations we find that the observed age spread is inconsistent with a coeval stellar population, but is in agreement with a star formation activity between 1.5 and 3.5 Myrs. We also observe light evidence for a trend of ages with spatial distribution.

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arXiv2010

The (sub-)millimeter SED of protoplanetary disks in the outskirts of the Orion Nebula Cluster

We present the sub-mm/mm SED for a sample of eight young circumstellar disks in the outer regions of the Orion Nebula Cluster. New observations were carried out at 2.9 mm with the CARMA array and for one disk, 216-0939, at 3.3 and 6.8 mm with ATCA. By combining these new millimeter data with literature measurements at sub-millimeter wavelengths we investigate grain growth and measure the dust mass in protoplanetary disks in the Orion Nebula Cluster. These data provide evidence for dust grain growth to at least millimeter-sizes for the first time in a high-mass star forming region. The obtained range in sub-mm/mm spectral index, namely 1.5-3.2, indicates that for disks in the outskirts of the Orion Nebula Cluster (projected distance from the cluster center between about 0.4 pc and 1.5 pc) grain growth to mm sizes occurs in the same manner as disks in regions where only low-mass stars form. Finally, in our sample three disks are more massive than about $0.05\,M_\odot$, confirming that massive disks are present in the outer regions of the Orion Nebula.

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arXiv2009

A Wide-Field Survey of the Orion Nebula Cluster in the Near-Infrared

We present J, H and K photometry of the Orion Nebula Cluster obtained at the CTIO/Blanco 4 m telescope in Cerro Tololo with the ISPI imager. From the observations we have assembled a catalog of about 7800 sources distributed over an area of approximately 30'x40', the largest of any survey deeper than 2MASS in this region. The catalog provides absolute coordinates accurate to about 0.15 arcseconds and 3sigma photometry in the 2MASS system down to J 19.5mag, H 18.0mag, K 18.5mag, enough to detect planetary size objects 1 Myr old under Av 10mag of extinction at the distance of the Orion Nebula. We present a preliminary analysis of the catalog, done comparing the (J-H, H-K) color-color diagram, the (H, J-H) and (K, H-K) color-magnitude diagrams and the JHK luminosity functions of three regions at increasing projected distance from the Trapezium. Sources in the inner region typically show IR colors compatible with reddened T Tauri stars, whereas the outer fields are dominated by field stars seen through an amount of extinction which decreases with the distance from the center. The color-magnitude diagrams make it possible to clearly distinguish between the main ONC population, spread across the full field, and background sources. The luminosity functions of the inner region, corrected for completeness, remain relatively flat in the sub-stellar regime regardless of the strategy adopted to remove background contamination.

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arXiv2009

Applications of DMDs for astrophysical research

A long-standing problem of astrophysical research is how to simultaneously obtain spectra of thousands of sources randomly positioned in the field of view of a telescope. Digital Micromirror Devices, used as optical switches, provide a most powerful solution allowing to design a new generation of instruments with unprecedented capabilities. We illustrate the key factors (opto-mechanical, cryo-thermal, cosmic radiation environment,...) that constrain the design of DMD-based multi-object spectrographs, with particular emphasis on the IR spectroscopic channel onboard the EUCLID mission, currently considered by the European Space Agency for a 2017 launch date.

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arXiv2008

Evidence for a Photoevaporated Circumbinary Disk in Orion

We have found a photoevaporated disk in the Orion Nebula that includes a wide binary. HST/ACS observations of the proplyd 124-132 show two point-like sources separated by 0".15, or about 60 AU at the distance of Orion. The two sources have nearly identical I and z magnitudes. We analyze the brightest component, Source N, comparing the observed magnitudes with those predicted using a 1 Myr Baraffe/NEXTGEN isochrone with different accretion luminosities and extinctions. We find that a low mass (\simeq 0.04 M_\odot) brown dwarf ~1 Myr old with mass accretion rate \log\dot{M}\simeq -10.3, typical for objects of this mass, and about 2 magnitudes of visual extinction provides the best fit to the data. This is the first observation of a circumbinary disk undergoing photoevaporation and, if confirmed by spectroscopic observations, the first direct detection of a wide substellar pair still accreting and enshrouded in its circumbinary disk.

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arXiv2008

SPACE: the SPectroscopic All-sky Cosmic Explorer

We describe the scientific motivations, the mission concept and the instrumentation of SPACE, a class-M mission proposed for concept study at the first call of the ESA Cosmic-Vision 2015-2025 planning cycle. SPACE aims to produce the largest three-dimensional evolutionary map of the Universe over the past 10 billion years by taking near-IR spectra and measuring redshifts for more than half a billion galaxies at 0<z<2 down to AB~23 over 3πsr of the sky. In addition, SPACE will also target a smaller sky field, performing a deep spectroscopic survey of millions of galaxies to AB~26 and at 2<z<10+. These goals are unreachable with ground-based observations due to the ~500 times higher sky background. To achieve the main science objectives, SPACE will use a 1.5m diameter Ritchey-Chretien telescope equipped with a set of arrays of Digital Micro-mirror Devices (DMDs) covering a total field of view of 0.4 deg2, and will perform large-multiplexing multi-object spectroscopy (e.g. ~6000 targets per pointing) at a spectral resolution of R~400 as well as diffraction-limited imaging with continuous coverage from 0.8mum to 1.8mum.

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arXiv2008

The enigma of GCIRS 3 - Constraining the properties of the mid-infrared reference star of the central parsec of the Milky Way with optical long baseline interferometry

GCIRS3 is the most prominent MIR source in the central pc of the Galaxy. NIR spectroscopy failed to solve the enigma of its nature. The properties of extreme individual objects of the central stellar cluster contribute to our knowledge of star and dust formation close to a supermassive black hole. We initiated an interferometric experiment to understand IRS3 and investigate its properties as spectroscopic and interferometric reference star at 10um. VISIR imaging separates a compact source from diffuse, surrounding emission. The VLTI/MIDI instrument was used to measure visibilities at 10mas resolution of that compact 10um source, still unresolved by a single VLT. Photometry data were added to enable simple SED- and full radiative transfer-models of the data. The luminosity and size estimates show that IRS3 is probably a cool carbon star enshrouded by a complex dust distribution. Dust temperatures were derived. The coinciding interpretation of multiple datasets confirm dust emission at several spatial scales. The IF data resolve the innermost area of dust formation. Despite observed deep silicate absorption towards IRS3 we favor a carbon rich chemistry of the circumstellar dust shell. The silicate absorption most probably takes place in the outer diffuse dust, which is mostly ignored by MIDI measurements. This indicates physically and chemically distinct conditions of the local dust, changing with the distance to IRS3. We have demonstrated that optical long baseline interferometry at infrared wavelengths is an indispensable tool to investigate sources at the Galactic Center. Our findings suggest further studies of the composition of interstellar dust and the shape of the 10um silicate feature at this outstanding region.

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arXiv2007

X-ray properties of protostars in the Orion Nebula

The origin and evolution of the X-rays in very young stellar objects (YSOs) are not yet well understood since it is very hard to observe YSOs in the protostellar phase. We study the X-ray properties of Class 0-I objects in the Orion Nebula Cluster (ONC) and compare them with those of the more evolved Class II and III members. Using Chandra Orion Ultradeep Project (COUP) data, we study the X-ray properties of stars in different evolutionary classes: luminosities, NH, temperatures and time variability are compared in order to understand if the interaction between the circumstellar material and the central object can influence the X-ray emission. We have assembled the deepest and most complete photometric catalog of objects in the ONC region from the UV to 8 microns using data from HST, WFI@2.2m ESO and ISPI@4m CTIO telescopes, and Spitzer IRAC. We select high probability candidate Class 0-I protostars, distinguishing between those having a spectral energy distribution which rises from K up to 8 microns (Class 0-Ia) from those where the SED rises from K up to 4.5 microns and decreasing afterwards (Class 0-Ib). We select a sample of bona fide Class II stars and a set of Class III stars with IR emission consistent with normal photospheres. Our principal result is that Class 0-Ia objects are significantly less luminous in X-rays, both in the total and hard bands, than the more evolved Class II stars with mass larger than 0.5 Msun; these latter show X-ray luminosities similar to those of Class 0-Ib stars. This result supports the hypothesis that the onset of X-ray emission occurs at a very early stage of star formation. Temporal variability and spectral properties of Class 0-I stars are similar to those of the more evolved Class II and III objects, except for a larger absorption likely due to gas in the circumstellar material.

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arXiv2007

An Overview of the SPACE Mission Proposal

SPACE (SPectroscopic All-sky Cosmic Explorer) is a class-M mission proposed to ESA for the Cosmic Vision 2015-2025 call and recently promoted to the next assessment study phase. SPACE will produce the first all-sky spectroscopic survey of the Universe, taking spectra of more than 500 million galaxies over a wide range of redshifts. SPACE will operate in slit mode (MEMS) at R~400 between 0.8 and 1.8micron down to AB~23, providing redshifts to an accuracy of Delta z~0.001, regardless on the presence of bright emission lines, together with the most relevant physical and evolutionary properties. The catalog of spectroscopic redshift will allow to place the ultimate constraints on the Baryon Acoustic Oscillations and the nature of Dark Energy. By obtaining the first 3-D all-sky map of the Universe at z~2 and beyond, SPACE will trace the growth rate of cosmic structures, the large scale structure of luminous baryons and its cosmic evolution. Besides the all-sky survey, SPACE will carry out a deep extragalactic survey over an area of ~10 sq. deg., enabling a most powerful supernova search program, and a galactic plane survey in integral field mode. Approximately 30% of the time will be open. Due to its versatility, SPACE is a ``self-sufficient'' observatory which can attack and solve the most compelling questions on the nature of the Dark Energy without complementary data from the Earth or space. Its unique wide-field capabilities in the near-IR make SPACE the ideal complement to JWST, ALMA, and the future 25-50 m telescopes.

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arXiv2007

A Hubble View of Star Forming Regions in the Magellanic Clouds

The Magellanic Clouds (MCs) offer an outstanding variety of young stellar associations, in which large samples of low-mass stars (with masses less than 1 solar mass) currently in the act of formation can be resolved and explored sufficiently with the Hubble Space Telescope. These pre-main sequence (PMS) stars provide a unique snapshot of the star formation process, as it is being recorded for the last 20 Myr, and they give important information on the low-mass Initial Mass Function (IMF) of their host environments. We present the latest results from observations with the Advanced Camera for Surveys (ACS) of such star-forming regions in the MCs, and discuss the importance of Hubble}for a comprehensive collection of substantial information on the most recent low-mass star formation and the low-mass IMF in the MCs.

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arXiv2005

A feasibility study of future observations with MIDI and other VLTI science instruments: The example of the Galactic Center

VLT-Interferometry will allow imaging of the Galactic Center (GC) and the nuclei of extragalactic sources at an angular resolution of a few milliarcseconds. VLTI will be a prime instrument to study the immediate environment of the massive black hole at the center of the Milky Way. With the MID infrared Interferometric instrument (MIDI) for example the enigmatic compact dust embedded MIR-excess sources within the central parsec should be resolvable. Further the observations of external galactic nuclei will allow unprecedented measurements of physical parameters (i.e. structure and luminosity) in these systems. With the exception of a few 'self-referencing' sources these faint-target observations will benefit from the available off-axis wavefront-correction system. To fully exploit the use of VLTI within this context, the following questions have to be addressed among others: How feasible is blind-pointing on (faint) science targets? Are VLTI observations still efficiently feasible if these faint science targets exceed the usual angular distance (<=1arcmin) to a GuideStar [...]? How is the fringe-tracking procedure affected in densely populated regions such as the GC? What preparatory steps have to be performed to successfully observe these non-standard targets with the VLTI? In this contribution, we present aspects for the preparation of VLTI observations, which will be conducted in the near future. Considering these example observations of the GC region, several details of observing modes are discussed, which are necessary to observe such science targets. The final goal is the definition of observational strategies [...] touching the limits of VLTI observability.

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arXiv2005

VLTI observations of IRS~3: The brightest compact MIR source at the Galactic Centre

The dust enshrouded star IRS~3 in the central light year of our galaxy was partially resolved in a recent VLTI experiment. The presented observation is the first step in investigating both IRS~3 in particular and the stellar population of the Galactic Centre in general with the VLTI at highest angular resolution. We will outline which scientific issues can be addressed by a complete MIDI dataset on IRS~3 in the mid infrared.

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arXiv2004

The Orion Nebula in the mid-IR

We present two wide-field (5'x5'), diffraction limited, broad-band 10 and 20 micron images of the Orion Nebula, plus six 7-13 micron narrow-band images of BN/KL taken at the 3.8m UKIRT telescope. The wide-field images are mosaics of 35"x35" frames reconstructed using a new restoration method developed for this project. They show the filamentary structure of the dust emission and reveal a new remarkable group of arc-like structures ~1' to the South of the Trapezium. The morphology of the Ney-Allen nebula suggests a complex kinematical status at the center of the Cluster. We find indications that the B0.5V star theta1 Ori-D, is surrounded by a photoevaporated circumstellar disk. Among the four historic Trapezium OB stars, this is the only one without a binary companion, suggesting that stellar multiplicity and the presence of massive circumstellar disks may be mutually exclusive. In the BN/KL complex we detect optically thin silicate emission on top of the deep 10 micron absorption feature. We map the foreground optical depth, color temperature and mid-IR luminosity of the embedded sources. We resolve a conspicuous point source at the location of the IRc2-A knot. The spectral profile of the 10 micron feature is compatible with grain crystallization. We detect several point sources In the OMC-1 South region and discuss their association with the mass loss observed at optical and mm wavelengths. Finally, we list the position and photometry of 177 point sources. With two possible exceptions, all point sources detected at shorter wavelengths display 10 micron emission well above the photospheric level. The model of Robberto et al. (2002, ApJ, 578, 897) provides the simplest explanation for the mid-IR excess.

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arXiv2004

Massive Star Feedback on the IMF

We have obtained the first measures of the mass accretion rates on stars of the Trapezium Cluster. They appear systematically lower than those of similar stars in the Taurus-Auriga association. Together with premature disk evaporation, dramatically revealed by the HST images of the Orion proplyds, this result suggests that low mass stars in a rich cluster may be ``dwarfed'' by the influence of nearby OB stars. This feedback mechanism affects the IMF, producing an excess of low mass stars and brown dwarfs. The observed frequency of low mass objects in Orion vs. Taurus seems to confirm this scenario.

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arXiv2004

HST/WFPC2 Study of the Trapezium Cluster: the Influence of Circumstellar Disks on the Initial Mass Function

We have performed the first measures of mass accretion rates in the core of the Orion Nebula Cluster. Four adjacent fields centered on the Trapezium stars have been imaged in the U- and B-bands using the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. In this paper we focus our attention on a group of 40 stars with known spectral types and complete UBVI WFPC2 photometry. Approximately three quarters of the sources show excess luminosity in the U-band, that we attribute to mass accretion. The known correlation between the U-band excess and the total accretion luminosity allows us to estimate the accretion rates. Overall, mass accretion rates appear lower than those measured by other authors in the Orion flanking fields or in Taurus-Auriga. Mass accretion rates remain low even in the vicinity of the birth line of Palla & Stahler, suggesting that in the core of the Trapezium cluster disk accretion has been recently depressed by an external mechanism. We suggest that the UV radiation generated by the Trapezium OB stars, responsible for the disk evaporation, may also cause the drop of the mass accretion rate. In this scenario, low-mass stars may terminate their pre-main sequence evolution with masses lower than those they would have reached if disk accretion could have proceeded undisturbed until the final disk consumption. In OB associations the low-mass end of the Initial Mass Function may therefore be affected by the rapid evolution of the most massive cluster's stars, causing a surplus "accretion aborted" very low-mass stars and brown dwarfs, and a deficit of intermediate mass stars. This trend is in agreement with recent observations of the IMF in the Trapezium cluster.

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arXiv2002

The Infrared Emission of Circumstellar Envelopes, Dark Silhouettes and Photoionized Disks in HII Regions

We model the IR Spectral Energy Distribution of circumstellar disks embedded in a HII region and photoevaporated by the external UV radiation. The model applies to the photoevaporated disks (proplyds) in Orion Nebula. First we calculate the IR emission of a PMS star surrounded by a dusty globule that is immersed within an HII region. The globule is assumed to be spherical, homogeneous, optically thin in the IR and photoevaporated according to the Dyson (1968) model. Second, we consider the IR emission of a disk directly exposed to the nebular environment. The disk is passive and treated according to the Chiang and Goldreich (1997) model. We improve over the CG97 treatment by tracing the propagation of the various radiative fluxes through the disk superheated atmosphere. Since the opposite disk sides receive different amounts of radiation, the flaring angle and the surface temperature distributions are different, resulting in well distinguished SEDs for the two disk faces. Finally, we combine the globule and disk models to estimate the IR emission of proplyds. The energy input from the central star and the nebular environment increase the disk flaring angle, and therefore also the amount of stellar radiation intercepted by the disk. The relative intensity of the disk vs. envelope emission varies with the tilt angle relative to the directions of th1Ori-C, and of the Earth. We explore the dependence of the SEDs upon the tilt angle with respect to the Earth, the distance from th1Ori-C, the size on the envelope, the inner disk radius and the temperature of the central star. The resulting SEDs are characterized by a broad peak of emission at 30-60micron, and are in general significantly different from those of isolated disks in low-mass star forming regions like Taurus-Auriga.

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arXiv2000

Wide-field imaging at mid infrared wavelengths: reconstruction of chopped and nodded data

Ground-based astronomical observations at thermal infrared wavelengths face the problem of extracting the weak astronomical signal from the large and rapidly variable background flux. The observing strategy most commonly used, the so-called ``chopping and nodding'' differential technique, provides reliable representations of the target uniquely in the case of compact sources while extended and complex sources can be easily distorted by their negative counterparts. A restoration method, designed to remove the negative values and to provide reliable representations of extended sources has been proposed. We show that the algorithm successfully removes the distortions due to the negative counterparts and, in addition, provides noise reduction.

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