Understanding how young stars gain their masses through disk-to-star accretion is of paramount importance in astrophysics. It affects our knowledge about the early stellar evolution, the disk lifetime and dissipation processes, the way the planets form on the smallest scales, or the connection to macroscopic parameters characterizing star-forming regions on the largest ones, among others. In turn, mass accretion rate estimates depend on the accretion paradigm assumed. For low-mass T Tauri stars with strong magnetic fields there is consensus that magnetospheric accretion (MA) is the driving mechanism, but the transfer of mass in massive young stellar objects with weak or negligible magnetic fields probably occurs directly from the disk to the star through a hot boundary layer (BL). The intermediate-mass Herbig Ae/Be (HAeBe) stars bridge the gap between both previous regimes and are still optically visible during the pre-main sequence phase, thus constituting a unique opportunity to test a possible change of accretion mode from MA to BL. This review deals with our estimates of accretion rates in HAeBes, critically discussing the different accretion paradigms. It shows that although mounting evidence supports that MA may extend to late-type HAes but not to early-type HBes, there is not yet a consensus on the validity of this scenario versus the BL one. Based on MA and BL shock modeling, it is argued that the ultraviolet regime could significantly contribute in the future to discriminating between these competing accretion scenarios.
Models of magnetically-driven accretion and outflows reproduce many observational properties of T Tauri stars. This concept is not well established for the more massive Herbig Ae/Be stars. We intend to examine the magnetospheric accretion in Herbig Ae/Be stars and search for rotational modulation using spectroscopic signatures, in this first paper concentrating on the well-studied Herbig Ae star HD101412. We used near-infrared spectroscopic observations of the magnetic Herbig Ae star HD101412 to test the magnetospheric character of its accretion disk/star interaction. We reduced and analyzed 3…
Models of magnetically driven accretion and outflows reproduce many observational properties of T Tauri stars. For the more massive Herbig Ae/Be stars, the corresponding picture is not well established. Nonetheless, it is expected that accretion flows in pre-main-sequence stars are guided from the circumstellar disk to stellar regions of high latitude along the magnetic field lines inside a magnetosphere. Using near-infrared multi-epoch spectroscopic data obtained with ISAAC, CRIRES, and X-shooter on the VLT, we examined magnetospheric accretion in the two Herbig Ae stars HD101412 and HD104237…
We investigate the physical properties of the inner gaseous disks of the three, hot, Herbig B2e stars, HD 76534, HD 114981 and HD 216629, by modelling CFHT-ESPaDOns spectra using non-LTE radiative transfer codes. We assume that the emission lines are produced in a circumstellar disk heated solely by the photospheric radiation from the central star in order to test if the optical and near-IR emission lines can be reproduced without invoking magnetospheric accretion. The inner gaseous disk density was assumed to follow a simple power-law in the equatorial plane, and we searched for models that c…
Herbig Ae/Be stars are young, pre-main-sequence stars that sample the transition in structure and evolution between low- and high-mass stars, providing a key test of accretion processes in higher-mass stars. Few Herbig Ae/Be stars have detected magnetic fields, calling into question whether the magnetospheric accretion paradigm developed for low-mass stars can be scaled to higher masses. We present He I 10830 Å line profiles for 64 Herbig Ae/Be stars with a magnetic field measurement in order to test magnetospheric accretion in the physical regime where its efficacy remains uncertain. Of the 5…
Herbig Ae/Be stars span a key mass range that links low and high mass stars, and thus provide an ideal window from which to explore their formation. This paper presents VLT/X-Shooter spectra of 91 Herbig Ae/Be stars, HAeBes; the largest spectroscopic study of HAeBe accretion to date. A homogeneous approach to determining stellar parameters is undertaken for the majority of the sample. Measurements of the ultra-violet (UV) are modelled within the context of magnetospheric accretion, allowing a direct determination of mass accretion rates. Multiple correlations are observed across the sample bet…
We review recent axisymmetric and three-dimensional (3D) magnetohydrodynamic (MHD) numerical simulations of magnetospheric accretion, plasma-field interaction and outflows from the disk-magnetosphere boundary.
We examine He I 10830 profile morphologies for a sample of 56 Herbig Ae/Be stars (HAEBES). We find significant differences between HAEBES and CTTSs in the statistics of both blue-shifted absorption (i.e. mass outflows) and red-shifted absorption features (i.e. mass infall or accretion). Our results suggest that, in general, Herbig Be (HBe) stars do not accrete material from their inner disks in the same manner as CTTSs, which are believed to accrete material via magnetospheric accretion, while Herbig Ae (HAe) stars generally show evidence for magnetospheric accretion. We find no evidence in ou…
This summary talk aims at highlighting some of the remaining open issues regarding the physics of the magnetospheric accretion/ejection process in young stellar objects (YSOs). It lists a number of questions related to YSOs magnetic fields and accretion regimes, the structure and properties of accretion shocks, the origin of multiple outflow components, and the impact of the star-disk magnetic interaction onto early angular momentum evolution.
This paper summarizes several results concerning the comparison between accretion-related properties of cool (T Tauri; T < 7000 K, M < 1 Msun and hot (Herbig Ae/Be; 7000 < T(K) < 13000; 1 < M(Msun) < 6) pre-main sequence (PMS) stars. This comparison gives insight into the analogies/differences in the physics of the star-disk interaction and in the physical mechanisms driving disk dissipation. Several optical and near-IR line luminosities used for low-mass objects are also valid to estimate typical accretion rates for intermediate-mass stars under similar empirical expressions. In contrast, the…
Magnetospheric accretion is an important process for a wide range of astrophysical systems, and may play a role in the formation of gas giant planets. Extending the formalism describing stellar magnetospheric accretion into the planetary regime, we demonstrate that magnetospheric processes may govern accretion onto young gas giants in the isolation phase of their development. Planets in the isolation phase have cleared out large gaps in their surrounding circumstellar disks, and settled into a quasi-static equilibrium with radii only modestly larger than their final sizes (i.e., $ r \sim 1.4 r…
We present the results of our observations of eight magnetic Herbig Ae/Be stars obtained with the X-shooter spectrograph mounted on UT2 at the VLT. X-shooter provides a simultaneous, medium-resolution and high-sensitivity spectrum over the entire wavelength range from 300 to 2500 nm. We estimate the mass accretion rates M_acc of the targets from 13 different spectral diagnostics using empiric calibrations derived previously for T Tauri-type stars and brown dwarfs. We have estimated the mass accretion rates of our targets, which range from 2x10^-9 to 2x10^-7 M_sun/yr. Furthermore, we have found…
The Herbig Ae/Be stars (HAeBes) allow an exploration of the properties of Pre-Main Sequence(PMS) stars above the low-mass range ($<2{\rm\thinspace M_{\odot}}$) and those bordering the high-mass range ($>8{\rm\thinspace M_{\odot}}$). This paper is the second in a series exploring accretion in 91 HAeBes with Very Large Telescope/X-shooter spectra. Equivalent width measurements are carried out on 32 different lines, spanning the UV to NIR, in order to obtain their line luminosities. The line luminosities were compared to accretion luminosities, which were determined directly from measurements of …
Models of magnetically driven accretion reproduce many observational properties of T Tauri stars. For the more massive Herbig Ae/Be stars, the corresponding picture has been questioned lately, in part driven by the fact that their magnetic fields are typically one order of magnitude weaker. Indeed, the search for magnetic fields in Herbig Ae/Be stars has been quite time consuming, with a detection rate of about 7%, also limited by the current potential to detect weak magnetic fields. Over the last two decades, magnetic fields were found in about twenty objects and for only two Herbig Ae/Be sta…
Accretion is the prime mode of star formation, but the exact mode has not yet been identified in the Herbig Ae/Be mass range. We provide evidence that the the maximum variation in mass-accretion rate is reached on a rotational timescale, which suggests that rotational modulation is the key to understanding mass accretion. We show how spectropolarimetry is uniquely capable of resolving the innermost (within 0.1 AU) regions between the star and the disk, allowing us to map the 3D geometry of the accreting gas, and test theories of angular momentum evolution. We present Monte Carlo line-emission …
Although the magnetospheric accretion model has been extensively applied to T Tauri Stars with typical mass accretion rates, the very low accretion regime is still not fully explored. Here we report multi-epoch observations and modeling of CVSO 1335, a 5 Myr old solar mass star which is accreting mass from the disk, as evidenced by redshifted absorption in the H$α$ profile, but with very uncertain estimates of mass accretion rate using traditional calibrators. We use the accretion shock model to constrain the mass accretion rate from the Balmer jump excess measured with respect to a non-accret…
The present study makes use of the unprecedented capability of the Gaia mission to obtain the stellar parameters such as distance, age, and mass of HAeBe stars. The accuracy of Gaia DR2 astrometry is demonstrated from the comparison of the Gaia DR2 distances of 131 HAeBe stars with the previously estimated values from the literature. This is one of the initial studies to estimate the age and mass of a confirmed sample of HAeBe stars using both the photometry and distance from the Gaia mission. Mass accretion rates are calculated from $Hα$ line flux measurements of 106 HAeBe stars. Since we use…
The dynamics of magnetic flux tubes (MFTs) in the accretion disk of typical Herbig Ae/Be star with fossil large-scale magnetic field is modeled taking into account the buoyant and drag forces, radiative heat exchange with the surrounding gas, and the magnetic field of the disk. The structure of the disk is simulated using our magnetohydrodynamic (MHD) model, taking into account the heating of the surface layers of the disk with the stellar radiation. The simulations show that MFTs periodically rise from the innermost region of the disk with speeds up to $10-12$ km s$^{-1}$. MFTs experience dec…
Understanding Balmer Decrements in T Tauri stars in terms of Multiflow Magnetospheric Accretion
Institute for Astrophysical Research, Department of Astronomy, Boston University（波士顿大学天体物理研究所）；Department of Astronomy, University of Michigan（密歇根大学天文学系）；Centro de Investigaciones de Astronomía (CIDA)（天文学研究中心）；Department of Physics, Faculty of Science, Chulalongkorn University（朱拉隆功大学理学院物理学系）
This work presents X-Shooter/VLT spectra of the prototypical, isolated Herbig Ae stars HD 31648 (MWC 480) and HD 163296 over five epochs separated by timescales ranging from days to months. Each spectrum spans over a wide wavelength range covering from 310 to 2475 nm. We have monitored the continuum excess in the Balmer region of the spectra and the luminosity of twelve ultraviolet, optical and near infrared spectral lines that are commonly used as accretion tracers for T Tauri stars. The observed strengths of the Balmer excesses have been reproduced from a magnetospheric accretion shock model…
Models of magnetically driven accretion and outflows reproduce many observational properties of T Tauri stars, but the picture is much less clear for the Herbig Ae/Be stars, due to the poor knowledge of their magnetic field strength and topology. The Herbig Ae star PDS2 was previously included in two magnetic studies based on low-resolution spectropolarimetric observations. Only in one of these studies the presence of a weak mean longitudinal magnetic field was reported. In the present study, for the first time, high-resolution HARPS spectropolarimetric observations of PDS2 are used to investi…
With the KOSMA 3-m telescope, 54 Herbig Ae/Be stars were surveyed in CO and $^{13}$CO emission lines. The properties of the stars and their circumstellar environments are studied by fitting the SEDs. The mean line width of $^{13}$CO (2-1) lines of this sample is 1.87 km s$^{-1}$. The average column density of H$_{2}$ is found to be $4.9\times10^{21}$ cm$^{-2}$ for the stars younger than $10^{6}$ yr, while drops to $2.5\times10^{21}$ cm$^{-2}$ for those older than $10^{6}$ yr. No significant difference is found among the SEDs of Herbig Ae stars and Herbig Be stars at the same age. The infrared …
Variability in emission lines is a characteristic feature in young stars and can be used as a tool to study the physics of the accretion process. Here we present a study of Hα variability in 15 T Tauri and Herbig Ae stars (K7-B2) over a wide range of time windows, from minutes, to hours, to days, and years. We assess the variability using linewidth measurements and the time series of line profiles. All objects show gradual, slow profile changes on time-scales of days. In addition, in three cases there is evidence for rapid variations in Hα with typical time-scales of 10 min, which occurs in 10…
We discuss results of global 3D MHD simulations of accretion on to a rotating magnetized star with a tilted dipole magnetic field, where the accretion is driven by the magneto-rotational instability (MRI). The simulations show that MRI-driven turbulence develops in the disc, and angular momentum is transported outwards due primarily to the magnetic stress. The turbulent flow is strongly inhomogeneous and the densest matter is in azimuthally-stretched turbulent cells. We investigate two regimes of accretion: a magnetospheric regime and a boundary layer (BL) regime. In the magnetospheric regime,…
Accretion onto Classical T Tauri stars is thought to take place through the action of magnetospheric processes, with gas in the inner disk being channeled onto the star's surface by the stellar magnetic field lines. Young stars are known to accrete material in a time-variable manner and the source of this variability remains an open problem, particularly on the shortest (~ day) timescales. Using one-dimensional time-dependent numerical simulations that follow the field line geometry, we find that for plausibly realistic young stars, steady-state transonic accretion occurs naturally in the abse…
We report on the status of our spectropolarimetric studies of Herbig Ae/Be stars carried out during the last years. The magnetic field geometries of these stars, investigated with spectropolarimetric time series, can likely be described by centred dipoles with polar magnetic field strengths of several hundred Gauss. A number of Herbig Ae/Be stars with detected magnetic fields have recently been observed with X-shooter in the visible and the near-IR, as well as with the high-resolution near-IR spectrograph CRIRES. These observations are of great importance to understand the relation between the…
We present Halpha linear spectropolarimetry of a large sample of Herbig Ae/Be stars. Together with newly obtained data for 17 objects, the sample contains 56 objects, the largest such sample to date. A change in linear polarization across the Halpha line is detected in 42 (75 %) objects, which confirms the previous finding that the circumstellar environment around these stars on small spatial scales has an asymmetric structure, which is typically identified with a disk. A second outcome of this research is that we confirm that Herbig Ae stars are similar to T Tauri stars in displaying a line p…
Recent observational work has indicated that mechanisms for accretion and outflow in Herbig Ae/Be star-disk systems may differ from magnetospheric accretion (MA) as it is thought to occur in T Tauri star-disk systems. In this work, we assess the temporal evolution of spectral lines probing accretion and mass loss in Herbig Ae/Be systems and test for consistency with the MA paradigm. For two Herbig Ae/Be stars, HD 98922 (B9e) and V1295 Aql (A2e), we have gathered multi-epoch (~years) and high-cadence (~minutes) high-resolution optical spectra to probe a wide range of kinematic processes. Employ…
Recent spectropolarimetric observations suggest that young low-mass stars such as classical T Tauri stars (CTTSs) possess relatively strong (~kG) magnetic field. This supports a scenario in which the final accretion onto the stellar surface proceeds through a magnetosphere, and the winds are formed in magnetohydrodynamics (MHD) processes. We examine recent numerical simulations of magnetospheric accretions via an inclined dipole and a complex magnetic fields. The difference between a stable accretion regime, in which accretion occurs in ordered funnel streams, and an unstable regime, in which …
Accretion plays an important role in protoplanetary disk evolution, and it is thought that the accretion mechanism changes between low- and high-mass stars. Here, we characterize accretion in intermediate-mass, pre-main-sequence Herbig Ae/Be (HAeBe) stars to search for correlations between accretion and system properties. We present new high-resolution, near-infrared spectra from the Immersion GRating INfrared Spectrograph for 102 HAeBes and analyze the accretion-tracing Br$γ$ line at 2.166 $μ$m. We also include the samples of Fairlamb et al. and Donehew & Brittain, for a total of 155 targets.…
We present an analytical formula for the position and shape of the spots on the surface of accreting magnetized stars in cases where a star has a dipole magnetic field tilted at a small misalignment angle Theta < 30 degrees about the rotational axis, and the magnetosphere is 2.5-5 times the radius of the star. We observed that the azimuthal position of the spots varies significantly when the position of the inner disc varies. In contrast, the polar position of the spots varies only slightly because of the compression of the magnetosphere. The azimuthal width of the spots strongly varies with T…
We present the results of a study of the temporal behaviour of several diagnostic lines formed in the region of the accretion-disk/star interaction in the three magnetic Herbig Ae stars HD101412, HD104237, and HD190073. More than 100 spectra acquired with the ISAAC, X-shooter, and CRIRES spectrographs installed at the VLT-8m telescope (ESO, Chile), as well as at other observatories (OHP, Crimean AO) were analyzed. The spectroscopic data were obtained in the He I lambda10830, Pa gamma and He I lambda5876 lines. We found that the temporal behaviour of the diagnostic lines in the spectra of all p…
Lithium and the evolution of intermediate-mass T Tauri and Herbig stars. Rotation, accretion, and planets
Centro de Astrobiología (CAB), CSIC-INTA（西班牙天体生物学中心）；European Southern Observatory（欧洲南方天文台）；INAF – Osservatorio Astronomico di Palermo（意大利国家天体物理研究所巴勒莫天文台）；Universidad de Granada（格拉纳达大学）；Universidad Autónoma de Madrid（马德里自治大学）；University of Dundee（邓迪大学）
This work aims to derive accretion rates for a sample of 38 HAeBe stars. We apply magnetospheric accretion (MA) shock modelling to reproduce the observed Balmer excesses. We look for possible correlations with the strength of the Halpha, [OI]6300, and Brgamma emission lines. The median mass accretion rate is 2 x 10^-7 Msun yr^-1 in our sample. The model fails to reproduce the large Balmer excesses shown by the four hottest stars (T* > 12000 K). We derive Macc propto M*^5 and Lacc propto L*^1.2 for our sample, with scatter. Empirical calibrations relating the accretion and the Halpha, [OI]6300,…
Among the 21 Herbig Ae/Be stars studied, new detections of a magnetic field were achieved in six stars. For three Herbig Ae/Be stars, we confirm previous magnetic field detections. The largest longitudinal magnetic field, <B_z> = -454+-42G, was detected in the Herbig Ae/Be star HD101412 using hydrogen lines. No field detection at a significance level of 3sigma was achieved in stars with debris disks. Our study does not indicate any correlation of the strength of the longitudinal magnetic field with disk orientation, disk geometry, or the presence of a companion. We also do not see any simple d…
Context. Classical T Tauri stars (cTTs) are pre-main sequence stars surrounded by an accretion disk. They host a strong magnetic field, and both magnetospheric accretion and ejection processes develop as the young magnetic star interacts with its disk. Studying this interaction is a major goal toward understanding the properties of young stars and their evolution. Aims. The goal of this study is to investigate the accretion process in the young stellar system HQ Tau, an intermediate-mass T Tauri star (1.9 M$_{\odot}$). Methods. The time variability of the system is investigated both photometri…
We present optical spectroscopy observations of 145 high-mass pre-main sequence candidates from the catalogue of Vioque et al. (2020). From these, we provide evidence for the Herbig nature of 128 sources. This increases the number of known objects of the class by $\sim50\%$. We determine the stellar parameters of these sources using the spectra and Gaia EDR3 data. The new sources are well distributed in mass and age, with 23 sources between $4$-$8$ M$_{\odot}$ and 32 sources above $8$ M$_{\odot}$. Accretion rates are inferred from H$α$ and H$β$ luminosities for 104 of the new Herbigs. These ac…
We perform three-dimensional magnetohydrodynamic simulations of magnetospheric accretion in a T Tauri star to study the accretion and wind structures in the close vicinity of the star. The gas accreting onto the star consists of the gas from the magnetospheric boundary and the failed disk winds. The accreting gas is commonly found as a multi-column accretion, which is consistent with observations. A significant fraction of the angular momentum of the accreting flows is removed by the magnetic fields of conical disk winds and turbulent failed winds inside and near the magnetosphere. As a result…
This work presents a spectroscopic study of 163 Herbig Ae/Be stars. Amongst these, we present new data for 30 objects. Stellar parameters such as temperature, reddening, mass, luminosity and age are homogeneously determined. Mass accretion rates are determined from $\rm Hα$ emission line measurements. Our data is complemented with the X-Shooter sample from previous studies and we update results using Gaia DR2 parallaxes giving a total of 78 objects with homogeneously determined stellar parameters and mass accretion rates. In addition, mass accretion rates of an additional 85 HAeBes are determi…
Young stars interact with their accretion disk through their strong magnetosphere. We investigate the magnetospheric accretion process in the young stellar system DoAr 44. We monitored the system over several rotational cycles, combining high-resolution optical and near-IR spectropolarimetry with long-baseline near-IR interferometry and multicolor photometry. DoAr 44 is a young 1.2 solar mass star, moderately accreting from its disk, and seen at a low inclination. We derive a rotational period of 2.96 d from the system's light curve. Several optical and near-IR line profiles probing the accret…
This paper examines the outflows associated with the interaction of a stellar magnetosphere with an accretion disk. In particular, we investigate the magnetospheric ejections (MEs) due to the expansion and reconnection of the field lines connecting the star with the disk. Our aim is to study the dynamical properties of the outflows and evaluate their impact on the angular momentum evolution of young protostars. Our models are based on axisymmetric time-dependent magneto-hydrodynamic simulations of the interaction of the dipolar magnetosphere of a rotating protostar with a viscous and resistive…
Young stellar objects are thought to accrete material from their circumstellar disks through their strong stellar magnetospheres. We aim to directly probe the magnetospheric accretion region on a scale of a few 0.01 au in a young stellar system using long-baseline optical interferometry. We observed the pre-transitional disk system DoAr 44 with VLTI/GRAVITY on two consecutive nights in the K-band. We computed interferometric visibilities and phases in the continuum and in the BrG line in order to constrain the extent and geometry of the emitting regions. We resolve the continuum emission of th…
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. We present high angular and spectral resolution observations of the Br Gamma emitting region in the Herbig Ae star HD163296 (MWC275) in order to probe the origin of this line and constrain the physical processes taking place at sub-AU scales in the circumstellar region. By means of VLTI-AMBER observations at high spectral resolution (R~12000), we studied interferometric visibilities, wavelength-differential…
We report here the first results of a multi-wavelength campaign focussing on magnetospheric accretion processes of the classical TTauri star (cTTS) V2129Oph. In this paper, we present spectropolarimetric observations collected in 2009 July with ESPaDOnS at the Canada-France-Hawaii Telescope (CFHT). Circularly polarised Zeeman signatures are clearly detected, both in photospheric absorption and accretion-powered emission lines, from time-series of which we reconstruct new maps of the magnetic field, photospheric brightness and accretion-powered emission at the surface of V2129Oph using our newe…
We examine a broad range of mass flow diagnostics in a large sample of Herbig Ae/Be stars (HAEBES) using high resolution optical spectra. The H-beta and He I 5876 angstrom lines show the highest incidence of P-Cygni (30%) and inverse P-Cygni (14%) morphologies, respectively. The Fe II 4924 angstrom line also shows a large incidence of P-Cygni profiles (11%). We find support for many of the conclusions reached in a study based on the analysis of the He I 10830 angstrom line in a large sample of HAEBES. Namely, HAEBES exhibit smaller fractions of both blue-shifted absorption (i.e. mass outflow) …
3D simulations of magnetospheric accretion in T Tauri stars: I. Disk truncation, stellar torques, and application to observations
National and Kapodistrian University of Athens（雅典国立卡波蒂斯坦大学）；Univ. Grenoble Alpes, CNRS, IPAG（格勒诺布尔阿尔卑斯大学，法国国家科学研究中心，IPAG）；INAF – Osservatorio Astrofisico di Torino（意大利国家天体物理研究所都灵天文台）
We present the results of a new magnetic field survey of Herbig Ae/Be and A debris disk stars. They are used to determine whether magnetic field properties in these stars are correlated with the mass-accretion rate, disk inclinations, companion(s), Silicates, PAHs, or show a more general correlation with age and X-ray emission as expected for the decay of a remnant dynamo.
Spectral observations of classical T Tauri stars show a wide range of line profiles, many of which reveal signs of matter inflow and outflow. Halpha is the most commonly observed line profile due to its intensity, and it is highly dependent on the characteristics of the surrounding environment of these stars. Our aim is to analyze how the Halpha line profile is affected by the various parameters of our model which contains both the magnetospheric and disk wind contributions to the Halpha flux. We used a dipolar axisymmetric stellar magnetic field to model the stellar magnetosphere and a modifi…
The VLT-FLAMES Tarantula Survey. XV. VFTS 822: a candidate Herbig B[e] star at low metallicity
Armagh Observatory（阿马天文台）；Department of Physcis & Astronomy, Queen’s University Belfast（贝尔法斯特女王大学物理与天文学系）；UK Astronomy Technology Centre, Royal Observatory, Edinburgh（爱丁堡皇家天文台英国天文学技术中心）；ESA/STScI（欧洲空间局/太空望远镜科学研究所）；Department of Physics and Astronomy, The Open University（开放大学物理与天文学系）；Astronomical Institute Anton Pannekoek, Amsterdam University（阿姆斯特丹大学安东·潘内库克天文研究所）；Institue voor Sterrenkunde, KU Leuven（鲁汶大学天体物理研究所）；European Space Astronomy Centre（欧洲空间天文中心）
AB Aur is a Herbig Ae star with a transitional disk. Transitional disks present substantial dust clearing in their inner regions, most probably because of the formation of one or more planets, although other explanations are still viable. In transitional objects, accretion is found to be about an order of magnitude smaller than in classical full disks. Since accretion is believed to be correlated with outflow activity, centimeter free-free jets are expected to be present in association with these systems, at weaker levels than in classical protoplanetary (full) systems. We present new observat…
Advances in high-resolution imaging have revealed H$α$ emission separated from the host star. It is generally believed that the emission is associated with forming planets in protoplanetary disks. However, the nature of this emission is still not fully understood. Here we report a modeling effort of H$α$ emission from the planets around the young star PDS 70. Using standard magnetospheric accretion models previously applied to accreting young stars, we find that the observed line fluxes can be reproduced using a range of parameters relevant to PDS 70b and c, with the mean mass accretion rate o…
Herbig Ae/Be stars lie in the mass range between low and high mass young stars, and therefore offer a unique opportunity to observe any changes in the formation processes that may occur across this boundary. This paper presents medium resolution VLT/X-Shooter spectra of six Herbig Ae/Be stars, drawn from a sample of 91 targets, and high resolution VLT/CRIRES spectra of five Herbig Ae/Be stars, chosen based on the presence of CO first overtone bandhead emission in their spectra. The X-Shooter survey reveals a low detection rate of CO first overtone emission (7 per cent), consisting of objects m…
This work presents CHARA/VEGA H$α$ spectro-interferometry (R ~ 6000, and $λ$/2B ~ 1 mas) of HD 179218 and HD 141569, doubling the sample of Herbig Ae/Be (HAeBe) stars for which this type of observations is available so far. The observed H$α$ emission is spatially unresolved, indicating that the size of the H$α$ emitting region is smaller than ~ 0.21 and 0.12 au for HD 179218 and HD 141529 (~ 15 and 16 R*, respectively). This is smaller than for the two other HAeBes previously observed with the same instrumentation. Two different scenarios have been explored in order to explain the compact line…
From observations collected with the ESPaDOnS & NARVAL spectropolarimeters at CFHT and TBL, we report the detection of Zeeman signatures on the prototypical classical TTauri star AATau, both in photospheric lines and accretion-powered emission lines. Using time series of unpolarized and circularly polarized spectra, we reconstruct at two epochs maps of the magnetic field, surface brightness and accretion-powered emission of AATau. We find that AATau hosts a 2-3kG magnetic dipole tilted at ~20deg to the rotation axis, and of presumably dynamo origin. We also show that the magnetic poles of AATa…
In recent years Herbig Ae/Be stars receive considerable attention as their disks are believed to be the sites of on-going planet formation. Confirming the presence of magnetic fields in these stars is critical for understanding the transport of angular momentum during the protostellar phase. Furthermore, magnetic fields set the conditions for strongly anisotropic accretion. In this study we present the results of our recent observing campaigns of a sample of Herbig Ae/Be stars aimed at measurements of their magnetic fields applying the Singular Value Decomposition method to high resolution spe…
