ABSTRACT
Radio-wavelength observations of protoplanetary discs can show whether large dust grains (pebbles) have formed on the pathway to aggregation of planetary cores. The 100-m Green Bank ...Telescope was used to make a four-subband (26–40 GHz) photometric survey of the Taurus and Ophiuchus regions, which is nearly complete for class II systems above fixed millimetre-flux thresholds. There is evidence of anomalous microwave emission in 40 per cent of the systems, indicating that radio observations of protoplanetary discs need good spectral coverage to distinguish the presence of dust. At most, one-quarter of the systems are seen to host pebbles, of radii as large as 1 cm. The lack of pebble-dominated systems suggests that this is a short-lived phase in particle size evolution, and/or that pebbles only grow in limited areas of the disc. Either case supports models where grains of centimetre size rapidly fragment and/or drift towards the star, potentially feeding growing planets. In the best-fitting systems, including the 26–40 GHz data raises the detected dust mass by up to an order of magnitude, and the mass distribution of the discs may be flatter. Both of these phenomena could help to solve the ‘missing mass’ problem, where the solid budget in protoplanetary discs is compared with the substantial requirements of extrasolar-planet systems.
Spiral density waves in a young protoplanetary disk Pérez, Laura M.; Carpenter, John M.; Andrews, Sean M. ...
Science (American Association for the Advancement of Science),
09/2016, Volume:
353, Issue:
6307
Journal Article
Peer reviewed
Open access
Gravitational forces are expected to excite spiral density waves in protoplanetary disks, disks of gas and dust orbiting young stars. However, previous observations that showed spiral structure were ...not able to probe disk midplanes, where most of the mass is concentrated and where planet formation takes place. Using the Atacama Large Millimeter/submillimeter Array, we detected a pair of trailing symmetric spiral arms in the protoplanetary disk surrounding the young star Elias 2-27. The arms extend to the disk outer regions and can be traced down to the midplane. These millimeter-wave observations also reveal an emission gap closer to the star than the spiral arms. We argue that the observed spirals trace shocks of spiral density waves in the midplane of this young disk.
Abstract
We present the first Atacama Large Millimeter/submillimeter Array (ALMA) observations of the closest known extrasolar debris disc. This disc orbits the star ε Eri, a K-type star just 3.2 pc ...away. Due to the proximity of the star, the entire disc cannot fit within the ALMA field of view. Therefore, the observations have been centred 18″ North of the star, providing us with a clear detection of the Northern arc of the ring, at a wavelength of 1.3 mm. The observed disc emission is found to be narrow with a width of just 11–13 AU. The fractional disc width we find is comparable to that of the Solar system’s Kuiper Belt and makes this one of the narrowest debris discs known. If the inner and outer edges are due to resonances with a planet then this planet likely has a semi-major axis of 48 AU. We find tentative evidence for clumps in the ring, although there is a strong chance that at least one is a background galaxy. We confirm, at much higher significance, the previous detection of an unresolved emission at the star that is above the level of the photosphere and attribute this excess to stellar chromospheric emission.
Searches for phosphine in Venus' atmosphere have sparked a debate. Cordiner et al. (2022, https://doi.org/10.1029/2022gl101055) analyze spectra from the Stratospheric Observatory For Infrared ...Astronomy (SOFIA) and infer <0.8 ppb of PH3. We noticed that some spectral artifacts arose from non‐essential calibration‐load signals. By‐passing these signals allows simpler post‐processing and a 5.7σ candidate detection, suggesting ∼3 ppb of PH3 above the clouds. Compiling six phosphine results hints at an inverted abundance trend: decreasing above the clouds but rising again in the mesosphere from some unexplained source. However, no such extra source is needed if phosphine is undergoing destruction by sunlight (photolysis), to a similar degree as on Earth. Low phosphine values/limits are found where the viewed part of the super‐rotating Venusian atmosphere had passed through sunlight, while high values are from views moving into sunlight. We suggest Venusian phosphine is indeed present, and so merits further work on models of its origins.
Plain Language Summary
Cordiner et al. (2022, https://doi.org/10.1029/2022gl101055) find no phosphine in Venus' atmosphere, using the airborne SOFIA telescope. By‐passing some instrumental effects, we extract a detection with 5.7σ‐confidence from the same data. We can resolve the tension between high and low PH3 abundance values by noticing that the former are from “mornings” in Venus' atmosphere and the latter from “evenings.” Sunlight reduces the amount of phosphine in Earth's atmosphere by an order of magnitude, so similarly on Venus, we might expect lower abundances in data taken when the part of the atmosphere observed has passed through sunlight. If the six available data sets can be reconciled in this way, further modeling of possible sources of PH3 (e.g., volcanic, disequilibrium chemistry, extant life) seem worthwhile.
Key Points
We recover Venusian phosphine in SOFIA spectra by reducing contaminating signals; the PH3 abundance is ∼3 part‐per billion (ppb)
Six recoveries/limits show PH3 depleting between clouds and mesosphere, which would require an unknown re‐formation process or extra source
Recoveries and upper limits can instead be reconciled by PH3 photolysis, as high/low abundances correspond to Venusian mornings/evenings
Interferometric Identification of a Pre-Brown Dwarf André, Philippe; Ward-Thompson, Derek; Greaves, Jane
Science (American Association for the Advancement of Science),
07/2012, Volume:
337, Issue:
6090
Journal Article
Peer reviewed
Open access
It is not known whether brown dwarfs stellar-like objects with masses less than the hydrogen-burning limit, 0.075 solar mass (MSymbol) are formed in the same way as solar-type stars or by some other ...process. Here we report the clear-cut identification of a self-gravitating condensation of gas and dust with a mass in the brown-dwarf regime, made through millimeter interferometric observations. The level of thermal millimeter continuum emission detected from this object indicates a mass -0.02 to 0.03 MSymbol, whereas the small radius, < 460 astronomical units, and narrow spectral lines imply a dynamical mass of 0.015 to 0.02 MSymbol. The identification of such a pre-brown dwarf core supports models according to which brown dwarfs are formed in the same manner as hydrogen-burning stars.
In light of recent substantial updates to spectral type estimations and newly established intrinsic colours, effective temperatures, and bolometric corrections for pre-main sequence (PMS) stars, we ...re-address the theory of accretion disc-regulated stellar angular momentum (AM) evolution. We report on the compilation of a consistent sample of fully convective stars within two of the most well-studied and youngest, nearby regions of star formation: the Orion nebula Cluster and Taurus–Auriga. We calculate the average specific stellar AM (j
⋆) assuming solid body rotation, using surface rotation periods gathered from the literature and new estimates of stellar radii and ages. We use published Spitzer IRAC fluxes to classify our stars as Class II or Class III and compare their j
⋆ evolution. Our results suggest that disc dispersal is a rapid process that occurs at a variety of ages. We find a consistent j
⋆ reduction rate between the Class II and Class III PMS stars which we interpret as indicating a period of accretion disc-regulated AM evolution followed by near-constant AM evolution once the disc has dissipated. Furthermore, assuming our observed spread in stellar ages is real, we find that the removal rate of j
⋆ during the Class II phase is more rapid than expected by contraction at constant stellar rotation rate. A much more efficient process of AM removal must exist, most likely in the form of an accretion-driven stellar wind or other outflow from the star–disc interaction region or extended disc surface.
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