12/03/2017

Catching up with the astro-ph: 02/01/2017

  • Polycyclic Aromatic Hydrocarbons in Protoplanetary Disks around Herbig Ae/Be and T Tauri Stars (Seok & Li, 2017) We find that the characteristic size of the PAHs tends to correlate with the stellar effective temperature ({T}{eff}) and interpret this as the preferential photodissociation of small PAHs in systems with higher {T}{eff} of which the stellar photons are more energetic. In addition, the PAH size shows a moderate correlation with the red-ward wavelength shift of the 7.7 μm PAH feature that is commonly observed in disks around cool stars. The ionization fraction of PAHs does not seem to correlate with any stellar parameters. This is because the charging of PAHs depends on not only the stellar properties (e.g., {T}{eff}, luminosity) but also their spatial distribution in the disks. The marginally negative correlation between PAH size and stellar age suggests that continuous replenishment of PAHs via the outgassing of cometary bodies and/or the collisional grinding of planetesimals and asteroids is required to maintain the abundance of small PAHs against complete destruction by photodissociation.

No interesting paper at astro-ph 03/01/2017 (rekt).

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11/03/2017

Catching up with the astro-ph: 02/01/2017

  • Detection of lithium in nearby young late-M dwarfs (Phan-Bao et al.)  We report here the first clear detections of lithium absorption in four targets and a marginal detection in one target. Our mass estimates indicate that two of them are young brown dwarfs, two are young brown dwarf candidates and one is a young very low-mass star. The closest young field brown dwarf in our sample at only 15 pc is an excellent benchmark for further studying physical properties of brown dwarfs in the range 100150 Myr. We did not detect any debris disks around three late-M dwarfs, and we estimated upper limits to the dust mass of debris disks around them. (at 850 microns, mind you)

Keeping up with the astro-ph: 10/03/2017

  • Constraining proto-planetary disc evolution using accretion rate and disc mass measurements: the usefulness of the dimensionless accretion parameter (Rosotti et al. ) We explore how measurements of protoplanetary disc masses and accretion rates […] can be analysed via the dimensionless accretion parameter, which we define as the product of the accretion rate and stellar age divided by the disc mass. […] we demonstrate that this parameter should be less than or of order unity (e.g. stationary state) for a wide range of evolutionary scenarios, rising above unity only during the final stages of outside in clearing by external photoevaporation. We find that while dust based mass estimates produce results compatible with theoretical expectations assuming canonical dust/gas ratio, the systematically lower CO based estimates yield accretion efficiencies significantly above unity in contrast with the theory. This finding provides additional evidence that CO based disc masses are an under-estimate […]we demonstrate that dust based mass estimates are sufficiently accurate to reveal distinctly higher accretion efficiencies in the Trapezium cluster, where this result is expected given the evident importance of external photoevaporation. We therefore propose the dimensionless accretion parameter as a new diagnostic of external photoevaporation in other star forming regions.