arXiv subjects

Luca Ricci

Publications and source records attributed to Luca Ricci.

79 records · Page 5Linked to original sources

arXiv2014

An ALMA Continuum Survey of Circumstellar Disks in the Upper Scorpius OB Association

We present ALMA 880 micron continuum observations of 20 K and M-type stars in the Upper Scorpius OB association that are surrounded by protoplanetary disks. These data are used to measure the dust content in disks around low mass stars (0.1-1.6 Msun) at a stellar age of 5-11 Myr. Thirteen sources were detected in the 880 micron dust continuum at >3 sigma with inferred dust masses between 0.3 and 52 Mearth. The dust masses tend to be higher around the more massive stars, but the significance is marginal in that the probability of no correlation is p ~ 0.03. The evolution in the dust content in disks was assessed by comparing the Upper Sco observations with published continuum measurements of disks around ~ 1-2 Myr stars in the Class II stage in the Taurus molecular cloud. While the dust masses in the Upper Sco disks are on average lower than in Taurus, any difference in the dust mass distributions is significant at less than 3sigma. For stellar masses between 0.49 Msun and 1.6 Msun, the mean dust mass in disks is lower in Upper Sco relative to Taurus by Delta log Mdust = 0.44 +/-0.26.

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arXiv2014

ALMA Observations of the Orion Proplyds

We present ALMA observations of protoplanetary disks ("proplyds") in the Orion Nebula Cluster. We imaged 5 individual fields at 856um containing 22 HST-identified proplyds and detected 21 of them. Eight of those disks were detected for the first time at submillimeter wavelengths, including the most prominent, well-known proplyd in the entire Orion Nebula, 114-426. Thermal dust emission in excess of any free-free component was measured in all but one of the detected disks, and ranged between 1-163 mJy, with resulting disk masses of 0.3-79 Mjup. An additional 26 stars with no prior evidence of associated disks in HST observations were also imaged within the 5 fields, but only 2 were detected. The disk mass upper limits for the undetected targets, which include OB stars, theta1Ori C and theta1Ori F, range from 0.1-0.6 Mjup. Combining these ALMA data with previous SMA observations, we find a lack of massive (>3 Mjup) disks in the extreme-UV dominated region of Orion, within 0.03 pc of O-star theta1Ori C. At larger separations from theta1Ori C, in the far-UV dominated region, there is a wide range of disk masses, similar to what is found in low-mass star forming regions. Taken together, these results suggest that a rapid dissipation of disk masses likely inhibits potential planet formation in the extreme-UV dominated regions of OB associations, but leaves disks in the far-UV dominated regions relatively unaffected.

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arXiv2014

Dust Evolution in Protoplanetary Disks

(abridged) In the core accretion scenario for the formation of planetary rocky cores, the first step toward planet formation is the growth of dust grains into larger and larger aggregates and eventually planetesimals. Although dust grains are thought to grow from the submicron sizes typical of interstellar dust to micron size particles in the dense regions of molecular clouds and cores, the growth from micron size particles to pebbles and kilometre size bodies must occur in protoplanetary disks. This step in the formation of planetary systems is the last stage of solids evolution that can be observed directly in young extrasolar systems. In this chapter we review the constraints on the physics of grain-grain collisions as they have emerged from laboratory experiments and numerical computations. We then review the current theoretical understanding of the global processes governing the evolution of solids in protoplanetary disks, including dust settling, growth, and radial transport. The predicted observational signatures are summarized. We discuss recent developments in the study of grain growth in molecular cloud cores and in collapsing envelopes of protostars as these provide the initial conditions for the dust in disks. We discuss the observational evidence for the growth of grains in young disks from mm surveys, as well as the recent evidence of radial variations of the dust properties in disks. We include a brief discussion of the constraints on the small end of the grain size distribution and on dust settling as derived from optical and IR observations. The observations are discussed in the context of global dust evolution models, in particular we focus on the emerging evidence for a very efficient early growth of grains and the radial distribution of grain sizes in disks. We also highlight the limits of current models, including the need to slow the radial drift of grains.

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arXiv2013

An azimuthal asymmetry in the LkHa 330 disk

Theory predicts that giant planets and low mass stellar companions shape circumstellar disks by opening annular gaps in the gas and dust spatial distribution. For more than a decade it has been debated whether this is the dominant process that leads to the formation of transitional disks. In this paper, we present millimeter-wave interferometric observations of the transitional disk around the young intermediate mass star LkHa330. These observations reveal a lopsided ring in the 1.3 mm dust thermal emission characterized by a radius of about 100 AU and an azimuthal intensity variation of a factor of 2. By comparing the observations with a Gaussian parametric model, we find that the observed asymmetry is consistent with a circular arc, that extends azimuthally by about 90 deg and emits about 1/3 of the total continuum flux at 1.3 mm. Hydrodynamic simulations show that this structure is similar to the azimuthal asymmetries in the disk surface density that might be produced by the dynamical interaction with unseen low mass companions orbiting within 70 AU from the central star. We argue that such asymmetries might lead to azimuthal variations in the millimeter-wave dust opacity and in the dust temperature, which will also affect the millimeter-wave continuum emission. Alternative explanations for the observed asymmetry that do not require the presence of companions cannot be ruled out with the existing data. Further observations of both the dust and molecular gas emission are required to derive firm conclusions on the origin of the asymmetry observed in the LkHa330 disk.

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arXiv2012

Constraints on the Radial Variation of Grain Growth in the AS 209 Circumstellar Disk

We present dust continuum observations of the protoplanetary disk surrounding the pre-main sequence star AS 209, spanning more than an order of magnitude in wavelength from 0.88 to 9.8 mm. The disk was observed with sub-arcsecond angular resolution (0.2"-0.5") to investigate radial variations in its dust properties. At longer wavelengths, the disk emission structure is notably more compact, providing model-independent evidence for changes in the grain properties across the disk. We find that physical models which reproduce the disk emission require a radial dependence of the dust opacity κ_ν. Assuming that the observed wavelength-dependent structure can be attributed to radial variations in the dust opacity spectral index (β), we find that β(R) increases from β<0.5 at \sim20 AU to β>1.5 for R>80 AU, inconsistent with a constant value of β across the disk (at the 10σ level). Furthermore, if radial variations of κ_ν are caused by particle growth, we find that the maximum size of the particle-size distribution (a_{max}) increases from sub-millimeter-sized grains in the outer disk (R>70 AU) to millimeter and centimeter-sized grains in the inner disk regions (R< 70 AU). We compare our observational constraint on a_{max}(R) with predictions from physical models of dust evolution in proto-planetary disks. For the dust composition and particle-size distribution investigated here, our observational constraints on a_{max}(R) are consistent with models where the maximum grain size is limited by radial drift.

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arXiv2012

Photoevaporation and spatial variation of grain sizes in Orion 114-426

Deep HST broad-band images taken with ACS and WFPC2 of the giant ($\sim 1000$ AU diameter) dark silhouette proplyd 114-426 in the Orion Nebula show that this system is tilted, asymmetric, warped and photoevaporated. The exquisite angular resolution of ACS allows us to map the distribution of dust grains at the northern translucent edge of the disk, dominated by the photoevaporative flow. Using the Mie theory for standard circumstellar disk grains, we find evidence for a spatial gradient in grain size. The typical dust radius, $\simeq 0.2-0.7 μ$m (less than what reported by previous studies) becomes smaller as the distance from the disk center increases, consistent with the expectations for the dynamic of dust entrained in a gaseous photoevaporative wind. Our analysis of the disk morphology and location within the nebula indicates that this system is photoevaporated by the diffuse radiation field of the Orion Nebula, while being shielded from the radiation coming directly from the central Trapezium stars. We estimate the mass-loss rate from the disk surface and the time-scale for total disk dissipation, which turns out to be of the order of $10^4$yr. Such a short time, of the order of 1/100 of the cluster age, indicates that this system is seen on the verge of destruction. This is compatible with the exceptional nature of the disk, namely its combination of huge size and low mass. Finally, we briefly discuss the viability of possible mechanisms that may lead to the peculiar morphology of this system: external UV flux, binary star and past close encounter.

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arXiv2008

The HST/ACS Atlas of Protoplanetary Disks in the Great Orion Nebula

We present the atlas of protoplanetary disks in the Orion Nebula based on the ACS/WFC images obtained for the HST Treasury Program on the Orion Nebula Cluster. The observations have been carried out in 5 photometric filters nearly equivalent to the standard B, V, Halpha, I, and z passbands. Our master catalog lists 178 externally ionized proto-planetary disks (proplyds), 28 disks seen only in absorption against the bright nebular background (silhouette disks), 8 disks seen only as dark lanes at the midplane of extended polar emission (bipolar nebulae or reflection nebulae) and 5 sources showing jet emission with no evidence of neither external ionized gas emission nor dark silhouette disks. Many of these disks are associated with jets seen in Halpha and circumstellar material detected through reflection emission in our broad-band filters; approximately 2/3 have identified counterparts in x-rays. A total of 47 objects (29 proplyds, 7 silhouette disks, 6 bipolar nebulae, 5 jets with no evidence of proplyd emission or silhouette disk) are new detections with HST. We include in our list 4 objects previously reported as circumstellar disks which have not been detected in our HST/ACS images either because they are hidden by the bleeding trails of a nearby saturated bright star or because of their location out of the HST/ACS Treasury Program field. Other 31 sources previously reported as extended objects do not harbor a stellar source in our HST/ACS images. We also report on the detection of 16 red, elongated sources. Their location at the edges of the field, far from the Trapezium Cluster core (> 10'), suggests that these are probably background galaxies observed through low extinction regions of the Orion Molecular Cloud OMC-1.

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