arXiv subjects

Ramiro Franco-Hernández

Publications and source records attributed to Ramiro Franco-Hernández.

3 recordsLinked to original sources

arXiv2026

Modeling the curved dust sublimation front in protoplanetary disks: a potential probe of midplane turbulence

We present a new approach to calculate the geometry and emission of the dust inner wall in disks around T Tauri stars. This calculation follows a self-consistent approach given the disk structure and adopts a density-dependent sublimation temperature for the dust. We built spectral energy distributions (SEDs) of disk models with curved walls around a $0.5\,M_\odot$ star, finding that the curved wall starts at a radius of $\sim 0.11$ au and extends to $\sim 0.38$ au. The dependence on mass accretion rate, dust settling, and disk inclination on the resulting SEDs is explored, as well as the impact of the height of the midplane layer containing large millimeter-sized grains. To test our models, we compare synthetic near-IR colors from a grid of disk models with observed colors for a large sample of disk-bearing T Tauri stars located in Taurus, IC 348, and the Orion complex. Most of the observed colors can be explained by combinations of mass accretion rates, dust settling, and inclinations within the expected ranges for T Tauri stars. However, populating the regions where observed JHK colors are most concentrated, requires the millimeter-size grains be spread up to 0.5--3 scale heights above the midplane. This result contradicts expectations of rapid dust settling and suggests a high degree of turbulence capable of lifting large grains toward the upper disk layers. These findings provide insight into the dynamical conditions of the disk midplane near the star.

astro-ph.SR↗

arXiv2022

A Census of the Low Accretors. II: Accretion Properties

Much is known about the processes driving accretion from protoplanetary disks onto low-mass pre-main-sequence stars (T Tauri stars). Nevertheless, it is unclear how accretion stops. To determine the accretion properties and their relation to stellar properties and gain insight into the last stages of accretion, we present a detailed analysis of 24 low and possible accretors, previously identified using the He I $λ$10830 line. We model moderate-resolution H$α$ profiles of these stars using magnetospheric accretion flow models that account for the chromospheric contribution at the line center. Based on parameters derived from the fits of 20 stars that can be reproduced with the models, we find a power-law relation between the disk truncation radius and the mass accretion rate consistent with predictions from theory and simulations. Comparing the corotation and truncation radii, we find that most of our targets are accreting in the unstable regime and rule out the propeller as the main process stopping accretion. For the truncation radius to be the same as the magnetic radius, the dipole magnetic field and/or the efficiency parameter $ξ$ need to be smaller than previously determined, suggesting that higher-order fields dominate in low accretion rates. Lastly, we determine that the lowest accretion rates that can be detected by H$α$ line modeling are $1-3\times10^{-11}M_{\odot}yr^{-1}$ for M3 stars and $3-5\times10^{-11}M_{\odot}yr^{-1}$ for K5 stars. These limits are lower than the observed accretion rates in our sample, suggesting that we have reached a physical lower limit. This limit, $\dot{M}\sim10^{-10}M_{\odot}yr^{-1}$, is consistent with EUV-dominated photoevaporation.

astro-ph.SR↗

arXiv2004

Time Variation in the Radio Flux Density from the Bipolar Ultracompact H II Region NGC 7538 IRS1

We present high angular resolution ($\sim0\rlap.{''}1$-$0\rlap.{''}4$) VLA observations at 2 and 6 cm made in 1983, 1986, and 1995 toward the ultracompact bipolar H II region NGC 7538 IRS1. We find, at both wavelengths, clear evidence of a decrease in the emission from the lobes. This decrease, of orden 20-30%, has not been observed previously in any ultracompact H II region. Most likely, it is due to recombination of the ionized gas in the lobes as a result of a decrease in the available ionizing photon flux. It is unclear if this decrease in the ionizing photon flux is due to an intrinsic change in the exciting star or to increased absorption of ionizing photons in the optically-thick core of the nebula.

astro-ph↗