New constraints on dust grain size and distribution in CQ Tau

Grain growth in circumstellar disks is expected to be the first step towards the formation of planetary systems. There is now evidence for grain growth in several disks around young stars. Radially resolved images of grain growth in circumstellar disks are believed to be a powerful tool to constrain the dust evolution models and the initial stage for the formation of planets. In this paper we attempt to provide these constraints for the disk surrounding the young star CQ Tau. This system was already suggested from previous studies to host a population of grains grown to large sizes. We present new high angular resolution (0.3-0.9 arcsec) observations at wavelengths from 850um to 3.6cm obtained at the SMA, IRAM-PdBI and NRAO-VLA interferometers. We perform a combined analysis of the spectral energy distribution and of the high-resolution images at different wavelengths using a model to describe the dust thermal emission from the circumstellar disk. We include a prescription for the gas emission from the inner regions of the system. We detect the presence of evolved dust by constraining the disk averaged dust opacity coefficient beta (computed between 1.3 and 7mm) to be 0.6+/-0.1. This confirms the earlier suggestions that the disk contains dust grains grown to significant sizes and puts this on firmer grounds by tightly constraining the gas contamination to the observed fluxes at mm-cm wavelengths. We report some evidence of radial variations in dust properties, but current resolution and sensitivity are still too low for definitive results.Comment: 9 pages, A&A in pres

CARMA interferometric observations of 2MASS J044427+2512: the first spatially resolved observations of thermal emission of a brown dwarf disk

We present CARMA 1.3 mm continuum data of the disk surrounding the young brown dwarf 2MASS J044427+2512 in the Taurus molecular cloud. The high angular resolution of the CARMA observations (0.16 arcsec) allows us to spatially resolve for the first time the thermal emission from dust around a brown dwarf. We analyze the interferometric visibilities and constrain the disk outer radius adopting disk models with power-law radial profiles of the dust surface density. In the case of a power-law index equal to or lower than 1, we obtain a disk radius in the range of about 15 - 30 AU, while larger disks are inferred for steeper radial profiles. By combining this information on the disk spatial extent with the sub-mm spectral index of this source we find conclusive evidence for mm-sized grains, or larger, in this brown dwarf disk. We discuss the implications of our results on the models of dust evolution in proto-planetary disks and brown dwarf formation.Comment: 14 pages, 3 figures, Accepted for publication in ApJ Letter

ALMA Observations of Circumstellar Disks in the Upper Scorpius OB Association

We present ALMA observations of 106 G-, K-, and M-type stars in the Upper Scorpius OB Association hosting circumstellar disks. With these data, we measure the 0.88 mm continuum and $^{12}$CO $J$ = 3$-$2 line fluxes of disks around low-mass ($0.14-1.66$ $M_{\odot}$) stars at an age of 5-11 Myr. Of the 75 primordial disks in the sample, 53 are detected in the dust continuum and 26 in CO. Of the 31 disks classified as debris/evolved transitional disks, 5 are detected in the continuum and none in CO. The lack of CO emission in approximately half of the disks with detected continuum emission can be explained if CO is optically thick but has a compact emitting area ($\lesssim 40$ au), or if the CO is heavily depleted by a factor of at least $\sim1000$ relative to interstellar medium abundances and is optically thin. The continuum measurements are used to estimate the dust mass of the disks. We find a correlation between disk dust mass and stellar host mass consistent with a power-law relation of $M_{\mathrm{dust}}\propto M_*^{1.67\pm0.37}$. Disk dust masses in Upper Sco are compared to those measured in the younger Taurus star-forming region to constrain the evolution of disk dust mass. We find that the difference in the mean of $\log(M_{\mathrm{dust}}/M_*)$ between Taurus and Upper Sco is $0.64\pm0.09$, such that $M_{\mathrm{dust}}/M_*$ is lower in Upper Sco by a factor of $\sim4.5$.Comment: 18 pages of text, 11 Figures, 5 Tables. Published in Ap

The effect of local optically thick regions in the long-wave emission of young circumstellar disks

Multi-wavelength observations of protoplanetary disks in the sub-millimeter continuum have measured spectral indices values which are significantly lower than what is found in the diffuse interstellar medium. Under the assumption that mm-wave emission of disks is mostly optically thin, these data have been generally interpreted as evidence for the presence of mm/cm-sized pebbles in the disk outer regions. In this work we investigate the effect of possible local optically thick regions on the mm-wave emission of protoplanetary disks without mm/cm-sized grains. A significant local increase of the optical depth in the disk can be caused by the concentration of solid particles, as predicted to result from a variety of proposed physical mechanisms. We calculate the filling factors and implied overdensities these optically thick regions would need to significantly affect the millimeter fluxes of disks, and we discuss their plausibility. We find that optically thick regions characterized by relatively small filling factors can reproduce the mm-data of young disks without requesting emission from mm/cm-sized pebbles. However, these optically thick regions require dust overdensities much larger than what predicted by any of the physical processes proposed in the literature to drive the concentration of solids. We find that only for the most massive disks it is possible and plausible to imagine that the presence of optically thick regions in the disk is responsible for the low measured values of the mm spectral index. For the majority of the disk population, optically thin emission from a population of large mm-sized grains remains the most plausible explanation. The results of this analysis further strengthen the scenario for which the measured low spectral indices of protoplanetary disks at mm wavelengths are due to the presence of large mm/cm-sized pebbles in the disk outer regions.Comment: 13 pages, 2 figures, A&A in pres

Brown dwarf disks with ALMA

We present ALMA continuum and spectral line data at 0.89 mm and 3.2 mm for three disks surrounding young brown dwarfs and very low mass stars in the Taurus star forming region. Dust thermal emission is detected and spatially resolved for all the three disks, while CO(J=3-2) emission is seen in two disks. We analyze the continuum visibilities and constrain the disks physical structure in dust. The results of our analysis show that the disks are relatively large, the smallest one with an outer radius of about 70 AU. The inferred disk radii, radial profiles of the dust surface density and disk to central object mass ratios lie within the ranges found for disks around more massive young stars. We derive from our observations the wavelength dependence of the millimeter dust opacity. In all the three disks data are consistent with the presence of grains with at least millimeter sizes, as also found for disks around young stars, and confirm that the early stages of the solid growth toward planetesimals occur also around very low mass objects. We discuss the implications of our findings on models of solids evolution in protoplanetary disks, on the main mechanisms proposed for the formation of brown dwarfs and very low mass stars, as well as on the potential of finding rocky and giant planets around very low mass objects.Comment: 15 pages, 10 figures, accepted for publication in Ap

ALMA observations of the debris disk around the young Solar Analog HD 107146

We present ALMA continuum observations at a wavelength of 1.25 mm of the debris disk surrounding the $\sim$ 100 Myr old solar analog HD 107146. The continuum emission extends from about 30 to 150 AU from the central star with a decrease in the surface brightness at intermediate radii. We analyze the ALMA interferometric visibilities using debris disk models with radial profiles for the dust surface density parametrized as i) a single power-law, ii) a single power-law with a gap, and iii) a double power-law. We find that models with a gap of radial width $\sim 8$ AU at a distance of $\sim 80$ AU from the central star, as well as double power-law models with a dip in the dust surface density at $\sim 70$ AU provide significantly better fits to the ALMA data than single power-law models. We discuss possible scenarios for the origin of the HD 107146 debris disk using models of planetesimal belts in which the formation of Pluto-sized objects trigger disruptive collisions of large bodies, as well as models which consider the interaction of a planetary system with a planetesimal belt and spatial variation of the dust opacity across the disk. If future observations with higher angular resolution and sensitivity confirm the fully-depleted gap structure discussed here, a planet with a mass of approximately a few Earth masses in a nearly circular orbit at $\sim 80$ AU from the central star would be a possible explanation for the presence of the gap.Comment: (38 pages, 7 figures, accepted for publication in ApJ

Measurement of Circumstellar Disk Sizes in the Upper Scorpius OB Association with ALMA

We present detailed modeling of the spatial distributions of gas and dust in 57 circumstellar disks in the Upper Scorpius OB Association observed with ALMA at sub-millimeter wavelengths. We fit power-law models to the dust surface density and CO $J$ = 3-2 surface brightness to measure the radial extent of dust and gas in these disks. We found that these disks are extremely compact: the 25 highest signal-to-noise disks have a median dust outer radius of 21 au, assuming an $R^{-1}$ dust surface density profile. Our lack of CO detections in the majority of our sample is consistent with these small disk sizes assuming the dust and CO share the same spatial distribution. Of seven disks in our sample with well-constrained dust and CO radii, four appear to be more extended in CO, although this may simply be due to higher optical depth of the CO. Comparison of the Upper Sco results with recent analyses of disks in Taurus, Ophiuchus, and Lupus suggests that the dust disks in Upper Sco may be $\sim3$ times smaller in size than their younger counterparts, although we caution that a more uniform analysis of the data across all regions is needed. We discuss the implications of these results for disk evolution.Comment: 15 pages of text, 7 figures, 3 tables. Accepted in Ap

Spin and energy relaxation in germanium studied by spin-polarized direct-gap photoluminescence

Spin orientation of photoexcited carriers and their energy relaxation is investigated in bulk Ge by studying spin-polarized recombination across the direct band gap. The control over parameters such as doping and lattice temperature is shown to yield high polarization degree, namely larger than 40%, as well as a fine-tuning of the angular momentum of the emitted light with a complete reversal between right- and left-handed circular polarization. By combining the measurement of the optical polarization state of band-edge luminescence and Monte Carlo simulations of carrier dynamics, we show that these very rich and complex phenomena are the result of the electron thermalization and cooling in the multi-valley conduction band of Ge. The circular polarization of the direct-gap radiative recombination is indeed affected by energy relaxation of hot electrons via the X valleys and the Coulomb interaction with extrinsic carriers. Finally, thermal activation of unpolarized L valley electrons accounts for the luminescence depolarization in the high temperature regime