Papers by Alessio Caratti o Garatti
Millimeter methanol emission in the high-mass young stellar object G24.33+0.14
Publications of the Astronomical Society of Japan
In 2019 September, a sudden flare of the 6.7 GHz methanol maser was observed toward the high-mass... more In 2019 September, a sudden flare of the 6.7 GHz methanol maser was observed toward the high-mass young stellar object (HMYSO) G24.33+0.14. This may represent the fourth detection of a transient mass accretion event in an HMYSO after S255IR NIRS3, NGC 6334I-MM1, and G358.93−0.03-MM1. G24.33+0.14 is unique among these sources as it clearly shows a repeating flare with an 8 yr interval. Using the Atacama Large Millimeter/submillimeter Array (ALMA), we observed the millimeter continuum and molecular lines toward G24.33+0.14 in the pre-flare phase in 2016 August (ALMA Cycle 3) and the mid-flare phase in 2019 September (ALMA Cycle 6). We identified three continuum sources in G24.33+0.14, and the brightest source, C1, which is closely associated with the 6.7 GHz maser emission, shows only a marginal increase in flux density with a flux ratio (Cycle 6$/$Cycle 3) of 1.16 ± 0.01, considering an additional absolute flux calibration uncertainty of $10\%$. We identified 26 transitions from 13 m...
Mid-Term Near-Infrared Variability of the Massive Young Stellar Object RAFGL 7009S associated with G25.65+1.05
The astronomer's telegram, Oct 1, 2017
Proceedings of European VLBI Network Mini-Symposium and Users' Meeting 2021 — PoS(EVN2021), Mar 30, 2022
The Maser Monitoring Organisation (M2O) is a research community of telescope operators, astronomy... more The Maser Monitoring Organisation (M2O) is a research community of telescope operators, astronomy researchers and maser theoreticians pursuing a joint goal of reaching a deeper understanding of maser emission and exploring its variety of uses as tracers of astrophysical events. These proceedings detail the origin, motivations and current status of the M2O, as was introduced at the 2021 EVN symposium.
arXiv (Cornell University), Oct 25, 2022
Context. The process of accretion in classical T Tauri stars (CTTSs) has been observed to vary on... more Context. The process of accretion in classical T Tauri stars (CTTSs) has been observed to vary on different timescales. Studying this variability is vital to understanding a star's evolution and provides insight into the complex processes at work within, including sources of the veiling present. Understanding the dichotomy between continuum veiling and emission line veiling is integral to accurately measuring the amount of veiling present in stellar spectra. Aims. Here, 15 roughly consecutive nights of optical spectroscopic data from the spectropolarimeter ESPaDOnS are utilised to characterise the short-term accretion activity in the CTTS, RU Lup, and investigate its relationship with the veiling in the Li i 6707 Å absorption line. Methods. The accretion-tracing H i Balmer series emission lines were studied and used to obtain the accretion luminosity (L acc) and mass accretion rate (Ṁ acc) for each night, which vary by a factor of ∼ 2 between the brightest and dimmest nights. We also measured the veiling using multiple photospheric absorption lines (Na i 5688 Å, Mn i 6021 Å, and Li i 6707 Å) for each night. Results. We find the Li i 6707 Å line provides measurements of veiling that produce a strong, positive correlation with L acc in the star. When corrected for Li depletion, the average veiling measured in the Li i 6707 Å line is r Li i, avg ∼ 3.25 ± 0.20, which is consistent with the other photospheric lines studied (r avg ∼ 3.28 ± 0.65). Conclusions. We measured short timescale variability in the accretion luminosity and mass accretion rate that are intrinsic and not due to geometric effects. As the forbidden line emission we observe ([O i] 6300 Å and [S ii] 6730 Å) remains remarkably constant over our epochs, it is clear that the variations in the mass accretion rate are too short to have an effect on these outflow tracers. Upon comparing the changes in veiling and accretion luminosity, we find a strong, positive correlation. This study provides an example of how this correlation can be used as a tool to determine whether a measured variability is due to extinction or an intrinsic change in accretion. As the determination of veiling is an independent process from measuring L acc , their relationship allows further exploration of accretion phenomena in young stars.
International audienceContext. η Car is one of the most intriguing luminous blue variables in the... more International audienceContext. η Car is one of the most intriguing luminous blue variables in the Galaxy. Observations and models of the X-ray, ultraviolet, optical, and infrared emission suggest a central binary in a highly eccentric orbit with a 5.54 yr period residing in its core. 2D and 3D radiative transfer and hydrodynamic simulations predict a primary with a dense and slow stellar wind that interacts with the faster and lower density wind of the secondary. The wind-wind collision scenario suggests that the secondary’s wind penetrates the primary’s wind creating a low-density cavity in it, with dense walls where the two winds interact. However, the morphology of the cavity and its physical properties are not yet fully constrained.Aims. We aim to trace the inner ∼5–50 au structure of η Car’s wind-wind interaction, as seen through Brγ and, for the first time, through the He I 2s-2p line.Methods. We have used spectro-interferometric observations with the K-band beam-combiner GRAV...
MIRI EC Protoplanetary and Debris Disks Survey
JWST Proposal. Cycle 1, Jul 1, 2017
PROJECT-J: PROtostellar JEt's Cradle Tested with JWST
JWST Proposal. Cycle 1, Mar 1, 2021
The Young Protostellar Outflow HH211
jwst, Jul 1, 2017
Astronomy & Astrophysics, 2017
Aims. Recently, substantial flaring in the 6.7 GHz methanol maser line has been observed toward t... more Aims. Recently, substantial flaring in the 6.7 GHz methanol maser line has been observed toward the high-mass young stellar object (YSO) S255 NIRS 3, where an accretion burst was also detected in the IR. Our goal is to study the change in the properties of the 6.7 GHz masers between the pre-and outburst phases, and investigate the connection between the maser and the accretion burst. Methods. With the Karl G. Jansky Very Large Array (JVLA) and the European VLBI Network (EVN), we performed observations of the 6.7 GHz masers (covering a range in angular resolution from a few milliarcseconds to ≈1) during the burst phase and compared these observations with pre-burst measurements at similar spatial scales. Results. The accretion burst and the subsequent increase in IR luminosity are very likely the origin of the 6.7 GHz maser flare. Since most maser centers operate in the unsaturated regime, a change by a relatively small factor (≈5) in the flux of pumping photons has produced an exponential growth in the maser intensity. The main pre-burst maser cluster is no longer detected during the burst. Compared to the pre-burst phase, flaring 6.7 GHz masers emit across a different V LSR range that is more strongly redshifted, and the emission extends over a larger area at larger separation from the high-mass YSO. In particular, the outburst peak emission originates from a remarkably extended (0. 2-0. 3) maser plateau at a radial distance of 500-1000 AU from the source. Conclusions. Both the maser flare and the extraordinarily large extent of the maser structure can be a natural consequence of the burst in the accretion luminosity of the high-mass YSO. Our results strongly support models that predict IR radiative pumping for the 6.7 GHz CH 3 OH masers.
Astronomy & Astrophysics, 2015
Context. Accretion and ejection are tightly connected and represent the fundamental mechanisms re... more Context. Accretion and ejection are tightly connected and represent the fundamental mechanisms regulating star formation. However, the exact physical processes involved are not yet fully understood. Aims. We present high angular and spectral resolution observations of the Brγ emitting region in the Herbig Ae star HD 163296 (MWC 275) in order to probe the origin of this line and constrain the physical processes taking place at sub-AU scales in the circumstellar region. Methods. By means of VLTI-AMBER observations at high spectral resolution (R ∼ 12 000), we studied interferometric visibilities, wavelength-differential phases, and closure phases across the Brγ line of HD 163296. To constrain the physical origin of the Brγ line in Herbig Ae stars, all the interferometric observables were compared with the predictions of a line radiative transfer disc wind model. Results. The measured visibilities clearly increase within the Brγ line, indicating that the Brγ emitting region is more compact than the continuum. By fitting a geometric Gaussian model to the continuum-corrected Brγ visibilities, we derived a compact radius of the Brγ emitting region of ∼0.07 ± 0.02 AU (Gaussian half width at half maximum; or a ring-fit radius of ∼0.08 ± 0.02 AU). To interpret the observations, we developed a magneto-centrifugally driven disc wind model. Our best disc wind model is able to reproduce, within the errors, all the interferometric observables and it predicts a launching region with an outer radius of ∼0.04 AU. However, the intensity distribution of the entire disc wind emitting region extends up to ∼0.16 AU. Conclusions. Our observations, along with a detailed modelling of the Brγ emitting region, suggest that most of the Brγ emission in HD 163296 originates from a disc wind with a launching region that is over five times more compact than previous estimates of the continuum dust rim radius.
Astronomy & Astrophysics
Aims. It is well known that low-mass young stellar objects (LMYSOs) gain a significant portion of... more Aims. It is well known that low-mass young stellar objects (LMYSOs) gain a significant portion of their final mass through episodes of very rapid accretion, with mass accretion rates up to Ṁ∗ ∼ 10−4 M⊙ yr−1. Recent observations of high-mass young stellar objects (HMYSOs) with masses M∗ ≳ 10 M⊙ uncovered outbursts with accretion rates exceeding Ṁ∗ ∼ 10−3 M⊙ yr−1. Here, we examine which scenarios proposed in the literature so far to explain accretion bursts of LMYSOs can also apply to the episodic accretion in HMYSOs. Methods. We utilise 1D time-dependent models of protoplanetary discs around HMYSOs to study burst properties. Results. We find that discs around HMYSOs are much hotter than those around their low-mass cousins. As a result, a much more extended region of the disc is prone to the thermal hydrogen ionisation and magnetorotational activation instabilities. The former, in particular, is found to be ubiquitous in a very wide range of accretion rates and disc viscosity paramete...
This short review examines the most recent observational results obtained for jets driven by high... more This short review examines the most recent observational results obtained for jets driven by high-mass young stellar objects (HMYSOs). Reviewed data range from near-infrared to radio wavelengths, with particular emphasis on the IR regime, and are presented along with perspectives for future observations achievable with newly available or forthcoming facilities.
Astronomy & Astrophysics, 2018
Context. Protostellar jets in high-mass young stellar objects (HMYSOs) play a key role in the und... more Context. Protostellar jets in high-mass young stellar objects (HMYSOs) play a key role in the understanding of star formation and provide us with an excellent tool to study fundamental properties of HMYSOs. Aims. We aim at studying the physical and kinematic properties of the near-infrared (NIR) jet of IRAS 13481-6124 from au to parsec scales. Methods. Our study includes NIR data from the Very Large Telescope instruments SINFONI, CRIRES, and ISAAC. Information about the source and its immediate environment is retrieved with SINFONI. The technique of spectro-astrometry is performed with CRIRES to study the jet on au scales. The parsec-scale jet and its kinematic and dynamic properties are investigated using ISAAC. Results. The SINFONI spectra in H and K bands are rich in emission lines that are mainly associated with ejection and accretion processes. Spectro-astrometry is applied to the Brγ line, and for the first time, to the Brα line, revealing their jet origin with milliarcsecond-...
The James Webb Space Telescope (JWST) is due for launch in 2020. Optimized for near to mid-infrar... more The James Webb Space Telescope (JWST) is due for launch in 2020. Optimized for near to mid-infrared wavelengths, it is the largest telescope ever put in space. One of the main science drivers is understanding star and planet formation: thus the study of outflows and disks is central not only to the JWST guaranteed time program but will also be the focus of many open time proposals. After briefly reviewing the four main instruments: NIRCAM, NIRSPEC, NIRISS and MIRI, the broad goals of the star and planetary formation guaranteed time programs will be outlined including the study of chemical processes in the inner disk, the effects of metallicity on accretion, and the atomic and molecular properties of embedded outflows.
Very low-mass Class I protostars have been investigated very little thus far. Variability of thes... more Very low-mass Class I protostars have been investigated very little thus far. Variability of these young stellar objects (YSOs) and whether or not they are capable of strong episodic accretion is also left relatively unstudied. We investigate accretion variability in IRS54, a Class I very low-mass protostar with a mass of M_⋆ 0.1 - 0.2 M_⊙. We obtained spectroscopic and photometric data with VLT/ISAAC and VLT/SINFONI in the near-infrared (J, H, and K bands) across four epochs (2005, 2010, 2013, and 2014). We used accretion-tracing lines (Paβ and Brγ) and outflow-tracing lines (H_2 and [FeII] to examine physical properties and kinematics of the object. A large increase in luminosity was found between the 2005 and 2013 epochs of more than 1 magnitude in the K band, followed in 2014 by a steep decrease. Consistently, the mass accretion rate (Ṁ_acc) rose by an order of magnitude from 10^-8 M_⊙ yr^-1 to 10^-7 M_⊙ yr^-1 between the two early epochs. The visual extinction (A_V) has a...
Abstract. We present results of AO-assisted K-band IFU spectroscopy of the mas-sive young star IR... more Abstract. We present results of AO-assisted K-band IFU spectroscopy of the mas-sive young star IRAS13481-6124 performed with ESO’s VLT/SINFONI instrument. Our spectro-astrometric analysis of the Brγ line revealed a photo-center shift with re-spect to the adjacent continuum of ∼1 AU at a distance of 3.1 kpc. The position angle of this shift matches with that of the outflow which confirms that the massive star is in-deed the driving source. Furthermore, a velocity gradient along the major disk axis was found which hints at the rotational sense of the ionized region, and thus of the disk as well. The gradient is not consistent with Keplerian motion but points to rigid rotation of the innermost disk. Notably, emission of H2 is absent from source while both shocked and fluorescent H2 emission are observed in its immediate surroundings. 1.
We present here the results of our recent SMA observations at 1.3 mm toward the high-mass star-fo... more We present here the results of our recent SMA observations at 1.3 mm toward the high-mass star-forming region (HMSFR) G23.01-0.41, with both the most extended and compact array configurations, providing sub-arcsecond and high sensitivity maps for different molecular lines (e.g., 12 CO and isotopomers, SiO, CH 3 CN, and CH 3 OH). We also complement this dataset with the spectral energy distribution (SED) between 3.4 μm and 1.1 mm (upper right panel) and the continuum images from the Hi-GAL/Herschel survey (upper left panel). The aim of these observations is twofold: 1) to image at high angular and spectral resolution the flattened, hot molecular core (HMC) detected toward G23.01-0.41, which contains strong masers and a radio continuum source (bottom panels); 2) to compare the spatial distribution and velocity field of the gas close to the central YSO with those of the associated molecular outflow (middle panels). The dust and molecular line emission trace a flattened structure inside a radius of 8000 AU from the center of radio continuum and maser line emission in the region. The equatorial plane of this HMC is strictly perpendicular to the main elongation of the outflow emission onto the plane of the sky, which extends over a 10 times larger region (~0.5 pc). The inner velocity field mapped with the CH 3 CN (12 K-11 K) lines outlines that molecular gas rotates about the outflow axis and is simultaneously dragged along the outflow direction indicating Hubble-law expansion. The IR SED from the HMC suggests the presence of a single O9.5 ZAMS star with a mass ~19 M๏, consistent with the mass required for centrifugal equilibrium.
Accretion Bursts from Young Stars
In recent years, episodic accretion in young stellar objects (YSOs) has gained increasing relevan... more In recent years, episodic accretion in young stellar objects (YSOs) has gained increasing relevance and the latest picture of YSO evolution suggests that a large portion of the accreted material is gathered during outbursts. Indeed, most recent observations have corroborated the idea that accretion bursts take place through a broad range of stellar masses, from very low mass to high-mass YSOs and at all stages of star formation. As a matter of fact, episodic accretion has challenged the traditional steady-state accretion model. Here, we review three recently discovered outbursting sources with outstanding characteristics, namely V2775 Ori, V960 Mon, and S255IR NIRS 3.
The methanol maser flare of S255IR and an outburst from the high-mass YSO S255IR-NIRS3 - more than a coincidence?
Uploads
Papers by Alessio Caratti o Garatti