arXiv subjects

Barbara Ercolano

Publications and source records attributed to Barbara Ercolano.

At least 55 records · Page 3Linked to original sources

arXiv2021

The dispersal of protoplanetary discs. II: Photoevaporation models with observationally derived irradiating spectra

Young solar-type stars are known to be strong X-ray emitters and their X-ray spectra have been widely studied. X-rays from the central star may play a crucial role in the thermodynamics and chemistry of the circumstellar material as well as in the atmospheric evolution of young planets. In this paper we present model spectra based on spectral parameters derived from the observations of young stars in the Orion Nebula Cluster from the Chandra Orion Ultradeep Project (COUP). The spectra are then used to calculate new photoevaporation prescriptions that can be used in disc and planet population synthesis models. Our models clearly show that disc wind mass loss rates are controlled by the stellar luminosity in the soft (100 eV - 1 keV) X- ray band. New analytical relations are provided for the mass loss rates and profiles of photoevaporative winds as a function of the luminosity in the soft X-ray band. The agreement between observed and predicted transition disc statistics moderately improved using the new spectra, but the observed population of strongly accreting large cavity discs can still not be reproduced by these models. Furthermore, our models predict a population of non-accreting transition discs that are not observed. This highlights the importance of considering the depletion of millimeter-sized dust grains from the outer disc, which is a likely reason why such discs have not been detected yet.

astro-ph.EP↗

arXiv2021

A dusty filament and turbulent CO spirals in HD135344B-SAO206462

Planet-disc interactions build up local pressure maxima that may halt the radial drift of protoplanetary dust, and pile it up in rings and crescents. ALMA observations of the HD135344B disc revealed two rings in the thermal continuum stemming from ~mm-sized dust. At higher frequencies the inner ring is brighter relative to the outer ring, which is also shaped as a crescent rather than a full ring. In near-IR scattered light images, the disc is modulated by a 2-armed grand-design spiral originating inside the ALMA inner ring. Such structures may be induced by a massive companion evacuating the central cavity, and by a giant planet in the gap separating both rings, that channels the accretion of small dust and gas through its filamentary wakes while stopping the larger dust from crossing the gap. Here we present ALMA observations in the J=(2-1)CO isotopologue lines and in the adjacent continuum, with up to 12km baselines. Angular resolutions of 0.03" reveal the tentative detection of a filament connecting both rings, and which coincides with a local discontinuity in the pitch angle of the IR spiral, proposed previously as the location of the protoplanet driving this spiral. Line diagnostics suggest that turbulence, or superposed velocity components, is particularly strong in the spirals. The 12CO(2-1) 3-D rotation curve points at stellocentric accretion at radii within the inner dust ring, with a radial velocity of up to ~6%+-0.5% Keplerian, which corresponds to an excessively large accretion rate of ~2E-6M_sun/yr if all of the CO layer follows the 12CO(2-1) kinematics. This suggests that only the surface layers of the disc are undergoing accretion, and that the line broadening is due to superposed laminar flows.

astro-ph.EP↗

arXiv2021

Presence of water on exomoons orbiting free-floating planets: a case study

A free-floating planet is a planetary-mass object that orbits around a non-stellar massive object (e.g. a brown dwarf) or around the Galactic Center. The presence of exomoons orbiting free-floating planets has been theoretically predicted by several models. Under specific conditions, these moons are able to retain an atmosphere capable of ensuring the long-term thermal stability of liquid water on their surface. We model this environment with a one-dimensional radiative-convective code coupled to a gas-phase chemical network including cosmic rays and ion-neutral reactions. We find that, under specific conditions and assuming stable orbital parameters over time, liquid water can be formed on the surface of the exomoon. The final amount of water for an Earth-mass exomonoon is smaller than the amount of water in Earth oceans, but enough to host the potential development of primordial life. The chemical equilibrium time-scale is controlled by cosmic rays, the main ionization driver in our model of the exomoon atmosphere.

astro-ph.EP↗

arXiv2021

The imprint of X-ray photoevaporation of planet-forming discs on the orbital distribution of giant planets -- II. Theoretical predictions

Numerical models have shown that disc dispersal via internal photoevaporation driven by the host star can successfully reproduce the observed pile-up of warm Jupiters near 1-2 au. However, since a range of different mechanisms have been proposed to cause the same feature, clear observational diagnostics of disc dispersal leaving an imprint in the observed distribution of giant planets could help to constrain the dominant mechanisms. We aim to assess the impact of disc dispersal via X-ray driven-photoevaporation (XPE) onto giant planet separations in order to provide theoretical constraints on the location and size of any possible features related to this process within their observed orbital distribution. For this purpose, we perform a set of 1D population syntheses with varying initial conditions and correlate the gas giants' final parking locations with the X-ray luminosities of their host stars in order to quantify observables of this process within the $L_\mathrm{x}$-$a$-plane of these systems. We find that XPE indeed creates an underdensity of gas giants near the gravitational radius, with corresponding pile-ups inside and/or outside of this location. However, the size and location of these features are strongly dependent on the choice of initial conditions in our model, such as the assumed formation location of the planets. XPE can strongly affect the migration process of giant planets and leave potentially observable signatures within the observed orbital separations of giant planets. However, due to the simplistic approach employed in our model, which lacks a self-consistent treatment of planet formation within an evolving disc, a quantitative analysis of the final planet population orbits is not possible. Our results however strongly motivate future studies to include realistic disc dispersal mechanisms into global planet population synthesis models.

astro-ph.EP↗

arXiv2021

Testing photoevaporation and MHD disk wind models through future high-angular resolution radio observations: the case of TW Hydrae

We present theoretical predictions for the free-free emission at cm wavelengths obtained from photoevaporation and MHD wind disk models adjusted to the case of the TW Hydrae young stellar object. For this system, disk photoevaporation with heating due to the high-energy photons from the star has been proposed as a possible mechanism to open the gap observed in the dust emission with ALMA. We show that the photoevaporation disk model predicts a radial profile for the free-free emission that is made of two main spatial components, one originated from the bound disk atmosphere at 0.5-1 au from the star, and another more extended component from the photoevaporative wind at larger disk radii. We also show that the stellar X-ray luminosity has a significant impact on both these components. The predicted radio emission from the MHD wind model has a smoother radial distribution which extends to closer distances to the star than the photoevaporation case. We also show that a future radio telescope such as the \textit{Next Generation Very Large Array} (ngVLA) would have enough sensitivity and angular resolution to spatially resolve the main structures predicted by these models.

astro-ph.EP↗

arXiv2021

Testing the models of X-ray driven photoevaporation with accreting stars in the Orion Nebula Cluster

Recent works highlight the importance of stellar X-rays on the evolution of the circumstellar disks of young stellar objects, especially for disk photoevaporation. A signature of this process may be seen in the so far tentatively observed dependence of stellar accretion rates on X-ray luminosities. According to models of X-ray driven photoevaporation, stars with higher X-ray luminosities should show lower accretion rates, on average, in a sample with similar masses and ages. To this aim, we have analyzed X-ray properties of young stars in the Orion Nebula Cluster determined with Chandra during the COUP observation as well as accretion data obtained from the photometric catalog of the HST Treasury Program. With these data, we have performed a statistical analysis of the relation between X-ray activity and accretion rates using partial linear regression analysis. The initial anticorrelation found with a sample of 332 young stars is considerably weaker compared to previous studies. However, excluding flaring activity or limiting the X-ray luminosity to the soft band (0.5 - 2.0 keV) leads to a stronger anticorrelation, which is statistically more significant. Furthermore, we have found a weak positive correlation between the higher component of the plasma temperature gained in the X-ray spectral fitting and the accretion rates, indicating that the hardness of the X-ray spectra may influence the accretion process. There is evidence for a weak anticorrelation, as predicted by theoretical models, suggesting that X-ray photoevaporation modulates the accretion rate through the inner disk at late stages of disk evolution, leading to a phase of photoevaporation-starved accretion.

astro-ph.SR↗

arXiv2021

Detection of new O-type stars in the obscured stellar cluster Tr 16-SE in the Carina Nebula with KMOS

The Carina Nebula harbors a large population of high-mass stars, including at least 75 O-type and Wolf-Rayet stars, but the current census is not complete since further high-mass stars may be hidden in or behind the dense dark clouds that pervade the association. With the aim of identifying optically obscured O- and early B-type stars in the Carina Nebula, we performed the first infrared spectroscopic study of stars in the optically obscured stellar cluster Tr 16-SE, located behind a dark dust lane south of eta Car. We used the integral-field spectrograph KMOS at the ESO VLT to obtain H- and K-band spectra with a resolution of R sim 4000 (Delta lambda sim 5 A) for 45 out of the 47 possible OB candidate stars in Tr 16-SE, and we derived spectral types for these stars. We find 15 stars in Tr 16-SE with spectral types between O5 and B2 (i.e., high-mass stars with M >= 8 Msun, only two of which were known before. An additional nine stars are classified as (Ae)Be stars (i.e., intermediate-mass pre-main-sequence stars), and most of the remaining targets show clear signatures of being late-type stars and are thus most likely foreground stars or background giants unrelated to the Carina Nebula. Our estimates of the stellar luminosities suggest that nine of the 15 O- and early B-type stars are members of Tr 16-SE, whereas the other six seem to be background objects. Our study increases the number of spectroscopically identified high-mass stars (M >= 8 Msun) in Tr 16-SE from two to nine and shows that Tr 16-SE is one of the larger clusters in the Carina Nebula. Our identification of three new stars with spectral types between O5 and O7 and four new stars with spectral types O9 to B1 significantly increases the number of spectroscopically identified O-type stars in the Carina Nebula.

astro-ph.GA↗

arXiv2021

Giant planet migration during the disc dispersal phase

Transition discs are expected to be a natural outcome of the interplay between photoevaporation (PE) and giant planet formation. Massive planets reduce the inflow of material from the outer to the inner disc, therefore triggering an earlier onset of disc dispersal due to PE through a process known as Planet-Induced PhotoEvaporation (PIPE). In this case, a cavity is formed as material inside the planetary orbit is removed by PE, leaving only the outer disc to drive the migration of the giant planet. We investigate the impact of PE on giant planet migration and focus specifically on the case of transition discs with an evacuated cavity inside the planet location. This is important for determining under what circumstances PE is efficient at halting the migration of giant planets, thus affecting the final orbital distribution of a population of planets. For this purpose, we use 2D FARGO simulations to model the migration of giant planets in a range of primordial and transition discs subject to PE. The results are then compared to the standard prescriptions used to calculate the migration tracks of planets in 1D planet population synthesis models. The FARGO simulations show that once the disc inside the planet location is depleted of gas, planet migration ceases. This contradicts the results obtained by the impulse approximation, which predicts the accelerated inward migration of planets in discs that have been cleared inside the planetary orbit. These results suggest that the impulse approximation may not be suitable for planets embedded in transition discs. A better approximation that could be used in 1D models would involve halting planet migration once the material inside the planetary orbit is depleted of gas and the surface density at the 3:2 mean motion resonance location in the outer disc reaches a threshold value of $0.01\,\mathrm{g\,cm^{-2}}$.

astro-ph.EP↗

arXiv2020

A highly non-Keplerian protoplanetary disc: Spiral structure in the gas disc of CQ Tau

In the past years, high angular resolution observations have revealed that circumstellar discs appear in a variety of shapes with diverse substructures being ubiquitous. This has given rise to the question of whether these substructures are triggered by planet-disc interactions. Besides direct imaging, one of the most promising methods to distinguish between different disc shaping mechanisms is to study the kinematics of the gas disc. In particular, the deviations of the rotation profile from Keplerian velocity can be used to probe perturbations in the gas pressure profile that may be caused by embedded planets. In this paper we aim to analyze the gas brightness temperature and kinematics of the transitional disc around the CQ Tau star in order to resolve and characterize substructure in the gas, caused by possible perturbers. For our analysis we use spatially resolved ALMA observations of 12CO, 13CO and C18O (J=2-1). We further extract robust line centroids for each channel map and fit a number of Keplerian disc models to the velocity field. The gas kinematics of the CQ Tau disc present non-Keplerian features, showing bent and twisted iso-velocity curves in 12CO and 13CO. Significant spiral structures are detected between 10-180 au in both the brightness temperature and the rotation velocity of 12CO after subtraction of an azimuthally symmetric model, which may be tracing planet-disc interactions with an embedded planet or low-mass companion. We identify three spirals, two in the brightness temperature and one in the velocity residuals, spanning a large azimuth and radial extent. The brightness temperature spirals are morphologically connected to spirals observed in NIR scattered light in the same disc, indicating a common origin. Together with the observed large dust and gas cavity, the spirals support the hypothesis of a massive embedded companion in the CQ Tau disc.

astro-ph.EP↗

arXiv2020

Solid accretion onto planetary cores in radiative disks

The solid accretion rate, necessary to grow gas giant planetary cores within the disk lifetime, has been a major constraint for theories of planet formation. We tested the solid accretion rate efficiency on planetary cores of different masses embedded in their birth disk, by means of 3D radiation-hydrodynamics, where we followed the evolution of a swarm of embedded solids of different sizes. We found that using a realistic equation of state and radiative cooling, the disk at 5 au is able to cool efficiently and reduce its aspect ratio. As a result, the pebble isolation mass is reached before the core grows to 10 Earth masses, stopping efficiently the pebble flux and creating a transition disk. Moreover, the reduced isolation mass halts the solid accretion before the core reaches the critical mass, leading to a barrier to giant planet formation, and it explains the large abundance of super-Earth planets in the observed population.

astro-ph.EP↗

arXiv2020

The interpretation of protoplanetary disc wind diagnostic lines from X-ray photoevaporation and analytical MHD models

High resolution spectra of typical wind diagnostics ([OI] 6300 Å and other forbidden emission lines) can often be decomposed into multiple components: high-velocity components with blueshifts up to several 100 km/s are usually attributed to fast jets, while narrow (NLVC) and broad (BLVC) low-velocity components are believed to trace slower disc winds. Under the assumption that the line-broadening is dominated by Keplerian rotation, several studies have found that the BLVCs should trace gas launched between 0.05 and 0.5 au and correlations between the properties of BLVCs and NLVCs have been interpreted as evidence for the emission tracing an extended MHD wind and not a photoevaporative wind. We calculated synthetic line profiles obtained from detailed photoionisation calculations of an X-ray photoevaporation model and a simple MHD wind model and analyzed the emission regions of different diagnostic lines and the resulting spectral profiles. The photoevaporation model reproduces most of the observed NLVCs but not the BLVCs or HVCs. The MHD model is able to reproduce all components but produces Keplerian double peaks at average inclinations that are rarely observed. The combination of MHD and photoevaporative winds could solve this problem. Our results suggest that the Gaussian decomposition does not allow for a clear distinction of flux from different wind regions and that the line broadening is often dominated by the velocity gradient in the outflow rather than by Keplerian rotation. We show that observed correlations between BLVC and NLVC do not necessarily imply a common origin in an extended MHD wind.

astro-ph.SR↗

arXiv2020

Extrasolar planets: from dust to new worlds

Thousands of exoplanets have been discovered and the search for life outside Earth is at the forefront of astrophysical research. The planets we observe show a mind-blowing diversity that current theories strive to explain as part of the quest to assess the chances of finding life outside the Earth.

physics.pop-ph↗

arXiv2019

The dispersal of protoplanetary discs I: A new generation of X-ray photoevaporation models

Photoevaporation of planet forming discs by high energy radiation from the central star is potentially a crucial mechanism for disc evolution and it may play an important role in the formation and evolution of planetary system. We present here a new generation of X-ray photoevaporation models for solar-type stars, based on a new set of hydrodynamical simulations, which account for stellar irradiation via a new, significantly improved, parameterisation of gas temperatures, based on detailed photoionisation and radiation transfer calculations. This is the first of a series of papers aiming at providing a library of models which cover the observed parameter space in stellar and disc mass, metallicity and stellar X-ray properties. We focus here on solar-type stars (0.7 M$_\odot$) with relatively low-mass discs (1% of the stellar mass) and explore the dependence of the wind mass loss rates on stellar X-ray luminosity. We model primordial discs as well as transition discs at various stages of evolution. Our 2D hydrodynamical models are then used to derive simple recipes for the mass loss rates that are suitable for inclusion in one-dimensional disc evolution and/or planet formation models typically employed for population synthesis studies. Line profiles from typical wind diagnostics ([OI] 6300 $\overset{\lower.5em\circ}{\mathrm{A}}$ and [NeII] 12.8 $μ$m) are also calculated for our models and found to be roughly in agreement with previous studies. Finally, we perform a population study of transition discs by means of one-dimensional viscous evolution models including our new photoevaporation prescription and find that roughly a half of observed transition discs cavities and accretion rates could be reproduced by our models.

astro-ph.EP↗

arXiv2019

Radiation-Hydrodynamical Models of X-ray Photoevaporation in Carbon Depleted Circumstellar Discs

The so-called transition discs provide an important tool to probe various mechanisms that might influence the evolution of protoplanetary discs and therefore the formation of planetary systems. One of these mechanisms is photoevaporation due to energetic radiation from the central star, which can in principal explain the occurrence of discs with inner cavities like transition discs. Current models, however, fail to reproduce a subset of the observed transition discs, namely objects with large measured cavities and vigorous accretion. For these objects the presence of (multiple) giant planets is often invoked to explain the observations. In our work we explore the possibility of X-ray photoevaporation operating in discs with different gas-phase depletion of carbon and show that the influence of photoevaporation can be extended in such low-metallicity discs. As carbon is one of the main contributors to the X-ray opacity, its depletion leads to larger penetration depths of X-rays in the disc and results in higher gas temperatures and stronger photoevaporative winds. We present radiation-hydrodynamical models of discs irradiated by internal X-ray+EUV radiation assuming Carbon gas-phase depletions by factors of 3,10 and 100 and derive realistic mass-loss rates and profiles. Our analysis yields robust temperature prescriptions as well as photoevaporative mass-loss rates and profiles which may be able to explain a larger fraction of the observed diversity of transition discs.

astro-ph.EP↗

arXiv2019

A dust and gas cavity in the disc around CQ Tau revealed by ALMA

The combination of high resolution and sensitivity offered by ALMA is revolutionizing our understanding of protoplanetary discs, as their bulk gas and dust distributions can be studied independently. In this paper we present resolved ALMA observations of the continuum emission ($λ=1.3$ mm) and CO isotopologues ($^{12}$CO, $^{13}$CO, C$^{18}$O $J=2-1$) integrated intensity from the disc around the nearby ($d = 162$ pc), intermediate mass ($M_{\star}=1.67\,M_{\odot}$) pre-main-sequence star CQ Tau. The data show an inner depression in continuum, and in both $^{13}$CO and C$^{18}$O emission. We employ a thermo-chemical model of the disc reproducing both continuum and gas radial intensity profiles, together with the disc SED. The models show that a gas inner cavity with size between 15 and 25 au is needed to reproduce the data with a density depletion factor between $\sim 10^{-1}$ and $\sim 10^{-3}$. The radial profile of the distinct cavity in the dust continuum is described by a Gaussian ring centered at $R_{\rm dust}=53\,$au and with a width of $σ=13\,$au. Three dimensional gas and dust numerical simulations of a disc with an embedded planet at a separation from the central star of $\sim20\,$au and with a mass of $\sim 6\textrm{-} 9\,M_{\rm Jup}$ reproduce qualitatively the gas and dust profiles of the CQ Tau disc. However, a one planet model appears not to be able to reproduce the dust Gaussian density profile predicted using the thermo-chemical modeling.

astro-ph.EP↗

arXiv2018

The imprint of X-ray photoevaporation of planet-forming discs on the orbital distribution of giant planets

High energy radiation from a planet host star can have strong influence on the final habitability of a system through several mechanisms. In this context we have constructed a catalogue containing the X-ray luminosities, as well as basic stellar and planetary properties of all known stars hosting giant planets (> 0.1 Mjup) that have been observed by the Chandra X-ray Observatory, XMM-Newton and/or ROSAT. Specifically in this paper we present a first application of this catalogue to search for a possible imprint of X-ray photoevaporation of planet-forming discs on the present-day orbital distribution of the observed giant planets. We found a suggestive void in the semi-major axis, a, versus X-ray luminosity, Lx, plane, roughly located between a ~ 0.05-1 au and Lx ~10^27-10^29 erg/s, which would be expected if photoevaporation played a dominant role in the migration history of these systems. However, due to the small observational sample size, the statistical significance of this feature cannot be proven at this point.

astro-ph.EP↗

arXiv2018

Dense gas kinematics and a narrow filament in the Orion A OMC1 region using NH3

We present combined observations of the NH3 (J,K) = (1,1) and (2,2) inversion transitions towards OMC1 in Orion A obtained by the Karl G. Jansky Very Large Array (VLA) and the 100 m Robert C. Byrd Green Bank Telescope (GBT). With an angular resolution of 6" (0.01 pc), these observations reveal with unprecedented detail the complex filamentary structure extending north of the active Orion BN/KL region in a field covering 6' x 7'. We find a 0.012 pc wide filament within OMC1, with an aspect ratio of ~37:1, that was missed in previous studies. Its orientation is directly compared to the relative orientation of the magnetic field from the James Clerk Maxwell Telescope BISTRO survey in Orion A. We find a small deviation of ~11 deg between the mean orientation of the filament and the magnetic field, suggesting that they are almost parallel to one another. The filament's column density is estimated to be 2-3 orders of magnitude larger than the filaments studied with Herschel and is possibly self-gravitating given the low values of turbulence found. We further produce maps of the gas kinematics by forward modeling the hyperfine structure of the NH3 (J,K) = (1,1) and (2,2) lines. The resulting distribution of velocity dispersions peaks at ~0.5 km/s, close to the subsonic regime of the gas. This value is about 0.2 km/s smaller than previously measured in single-dish observations of the same region, suggesting that higher angular and spectral resolution observations will identify even lower velocity dispersions that might reach the subsonic turbulence regime in dense gas filaments.

astro-ph.GA↗

arXiv2018

The comparative effect of FUV, EUV and X-ray disc photoevaporation on gas giant separations

Gas giants' early ($\lesssim 5$ Myr) orbital evolution occurs in a disc losing mass in part to photoevaporation driven by high energy irradiance from the host star. This process may ultimately overcome viscous accretion to disperse the disc and halt migrating giants by starving their orbits of gas, imprinting on giant planet separations in evolved systems. Inversion of this distribution could then give insight into whether stellar FUV, EUV or X-ray flux dominates photoevaporation, constraining planet formation and disc evolution models. We use a 1D hydrodynamic code in population syntheses for gas giants undergoing Type II migration in a viscously evolving disc subject to either a primarily FUV, EUV or X-ray flux from a pre-solar T Tauri star. The photoevaporative mass loss profile's unique peak location and width in each energetic regime produces characteristic features in the distribution of giant separations: a severe dearth of $\lesssim$ 2 M$_{\rm J}$ planets interior to 5 AU in the FUV scenario, a sharp concentration of $\lesssim$ 3 M$_{\rm J}$ planets between $\approx 1.5 - 2$ AU in the EUV case, and a relative abundance of $\approx 2 - 3.5$ M$_{\rm J}$ giants interior to 0.5 AU in the X-ray model. These features do not resemble the observational sample of gas giants with mass constraints, though our results do show some weaker qualitative similarities. We thus assess how the differing photoevaporative profiles interact with migrating giants and address the effects of large model uncertainties as a step to better connect disc models with trends in the exoplanet population.

astro-ph.EP↗