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تسجيل مستخدم جديدA bright inner disk and structures in the transition disk around the very low-mass star CIDA 1
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Observations of protoplanetary disks around very low-mass stars and brown dwarfs remain challenging and little is known about their properties. The disk around CIDA1 ($sim$0.1-0.2$M_odot$) is one of the very few known disks that host a large cavity (20au radius in size) around a very low-mass star. We present new ALMA observations at Band7 (0.9mm) and Band4 (2.1mm) of CIDA1 with a resolution of $sim 0.05times 0.034$. These new ALMA observations reveal a very bright and unresolved inner disk, a shallow spectral index of the dust emission ($sim2$), and a complex morphology of a ring located at 20au. We also present X-Shooter (VLT) observations that confirm the high accretion rate of CIDA1 of $dot{M}_{rm acc}$=1.4 $times~10^{-8}M_odot$/yr. This high value of $dot{M}_{rm acc}$, the observed inner disk, and the large cavity of 20au exclude models of photo-evaporation to explain the observed cavity. When comparing these observations with models that combine planet-disk interaction, dust evolution, and radiative transfer, we exclude planets more massive than 0.5$M_{rm{Jup}}$ as the potential origin of the large cavity because with these it is difficult to maintain a long-lived and bright inner disk. Even in this planet mass regime, an additional physical process may be needed to stop the particles from migrating inwards and to maintain a bright inner disk on timescales of millions of years. Such mechanisms include a trap formed by a very close-in extra planet or the inner edge of a dead zone. The low spectral index of the disk around CIDA1 is difficult to explain and challenges our current dust evolution models, in particular processes like fragmentation, growth, and diffusion of particles inside pressure bumps.
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Context. Transition disks (TDs) are circumstellar disks with inner regions highly depleted in dust. TDs are observed in a small fraction of disk-bearing objects at ages of 1-10 Myr. They are important laboratories to study evolutionary effects in dis
ks, from photoevaporation to planet-disk interactions. Aims. We report the discovery of a large inner dust-empty region in the disk around the very low mass star CIDA 1 (M$_{star} sim 0.1-0.2$ M$_{odot}$). Methods. We used ALMA continuum observations at 887$mu$m, which provide a spatial resolution of $0.21times0.12$ ($sim$15$times$8 au in radius at 140 pc). Results. The data show a dusty ring with a clear cavity of radius $sim$20 au, the typical characteristic of a TD. The emission in the ring is well described by a narrow Gaussian profile. The dust mass in the disk is $sim$17 M$_{oplus}$. CIDA 1 is one of the lowest mass stars with a clearly detected millimeter cavity. When compared to objects of similar stellar mass, it has a relatively massive dusty disk (less than $sim5$% of Taurus Class II disks in Taurus have a ratio of $M_{rm{disk}}/M_{star}$ larger than CIDA 1) and a very high mass accretion rate (CIDA 1 is a disk with one of the lowest values of $M_{rm{disk}}/dot M$ ever observed). In light of these unusual parameters, we discuss a number of possible mechanisms that can be responsible for the formation of the dust cavity (e.g., photoevaporation, dead zones, embedded planets, close binary). We find that an embedded planet of a Saturn mass or a close binary are the most likely possibilities.
We aim to estimate if structures, such as cavities, rings, and gaps, are common in disks around VLMS and to test models of structure formation in these disks. We also aim to compare the radial extent of the gas and dust emission in disks around VLMS,
which can give us insight about radial drift. We studied six disks around VLMS in the Taurus star-forming region using ALMA Band 7 ($sim 340,$GHz) at a resolution of $sim0.1$. The targets were selected because of their high disk dust content in their stellar mass regime. Our observations resolve the disk dust continuum in all disks. In addition, we detect the $^{12}$CO ($J=3-2$) emission line in all targets and $^{13}$CO ($J=3-2$) in five of the six sources. The angular resolution allows the detection of dust substructures in three out of the six disks, which we studied by using UV-modeling. Central cavities are observed in the disks around stars MHO,6 (M5.0) and CIDA,1 (M4.5), while we have a tentative detection of a multi-ringed disk around J0433. Single planets of masses $0.1sim0.4,M_{rm{Jup}}$ would be required. The other three disks with no observed structures are the most compact and faintest in our sample. The emission of $^{12}$CO and $^{13}$CO is more extended than the dust continuum emission in all disks of our sample. When using the $^{12}$CO emission to determine the gas disk extension $R_{rm{gas}}$, the ratio of $R_{rm{gas}}/R_{rm{dust}}$ in our sample varies from 2.3 to 6.0, which is consistent with models of radial drift being very efficient around VLMS in the absence of substructures. Our observations do not exclude giant planet formation on the substructures observed. A comparison of the size and luminosity of VLMS disks with their counterparts around higher mass stars shows that they follow a similar relation.
We report the discovery of a scattering component around the HD 141569 A circumstellar debris system, interior to the previously known inner ring. The discovered inner disk component, obtained in broadband optical light with HST/STIS coronagraphy, wa
s imaged with an inner working angle of 0.25, and can be traced from 0.4 (~46 AU) to 1.0 (~116 AU) after deprojection using i=55deg. The inner disk component is seen to forward scatter in a manner similar to the previously known rings, has a pericenter offset of ~6 AU, and break points where the slope of the surface brightness changes. It also has a spiral arm trailing in the same sense as other spiral arms and arcs seen at larger stellocentric distances. The inner disk spatially overlaps with the previously reported warm gas disk seen in thermal emission. We detect no point sources within 2 (~232 AU), in particular in the gap between the inner disk component and the inner ring. Our upper limit of 9+/-3 M_J is augmented by a new dynamical limit on single planetary mass bodies in the gap between the inner disk component and the inner ring of 1 M_J, which is broadly consistent with previous estimates.
We present CARMA 1.3 mm continuum observations of the T Tauri star LkCa 15,which resolve the circumstellar dust continuum emission on angular scales between 0.2-3 arcsec, corresponding to 28-420 AU at the distance of the star. The observations resolv
e the inner gap in the dust emission and reveal an asymmetric dust distribution in the outer disk. (Abridge) We calculate that 90% of the dust emission arises from an azimuthally symmetric ring that contains about 5x10^{-4} M_sun of dust. A low surface-brightness tail that extends to the northwest out to a radius of about 300 AU contains the remaining 10% of the observed continuum emission. The ring is modeled with a rather flat surface density profile between 40 and 120 AU, while the inner cavity is consistent with either a sharp drop of the 1.3 mm dust optical depth at about 42 AU or a smooth inward decrease between 3 and 85 AU. (Abridge). Within 40 AU, the observations constrain the amount of dust between 10^{-6} and 7 Earth masses, where the minimum and maximum limits are set by the near-IR SED modeling and by the mm-wave observations of the dust emission respectively. In addition, we confirm the discrepancy in the outer disk radius inferred from the dust and gas, which corresponds to 150 AU and 900 AU respectively. We cannot reconcile this difference by adopting an exponentially tapered surface density profile as suggested for other systems, but we instead suggest that the gas surface density in the outer disk decreases less steeply than that predicted by model fits to the dust continuum emission. The lack of continuum emission at radii lager than 120 AU suggests a drop of at least a factor of 5 in the dust-to-gas ratio, or in the dust opacity. We show that a sharp dust opacity drop of this magnitude is consistent with a radial variation of the grain size distribution as predicted by existing grain growth models.
We compare line emission calculated from theoretical disk models with optical to sub-millimeter wavelength observational data of the gas disk surrounding TW Hya and infer the spatial distribution of mass in the gas disk. The model disk that best matc
hes observations has a gas mass ranging from $sim10^{-4}-10^{-5}$ms for $0.06{rm AU} <r<3.5$AU and $sim 0.06$ms for $ 3.5 {rm AU} <r<200$AU. We find that the inner dust hole ($r<3.5$AU) in the disk must be depleted of gas by $sim 1-2$ orders of magnitude compared to the extrapolated surface density distribution of the outer disk. Grain growth alone is therefore not a viable explanation for the dust hole. CO vibrational emission arises within $rsim 0.5$AU from thermal excitation of gas. [OI] 6300AA and 5577AA forbidden lines and OH mid-infrared emission are mainly due to prompt emission following UV photodissociation of OH and water at $rlesssim0.1$AU and at $rsim 4$AU. [NeII] emission is consistent with an origin in X-ray heated neutral gas at $rlesssim 10$AU, and may not require the presence of a significant EUV ($h u>13.6$eV) flux from TW Hya. H$_2$ pure rotational line emission comes primarily from $rsim 1-30$AU. [OI]63$mu$m, HCO$^+$ and CO pure rotational lines all arise from the outer disk at $rsim30-120$AU. We discuss planet formation and photoevaporation as causes for the decrease in surface density of gas and dust inside 4 AU. If a planet is present, our results suggest a planet mass $sim 4-7$M$_J$ situated at $sim 3$AU. Using our photoevaporation models and the best surface density profile match to observations, we estimate a current photoevaporative mass loss rate of $4times10^{-9}$ms yr$^{-1}$ and a remaining disk lifetime of $sim 5$ million years.
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