Accretion rates and accretion tracers of Herbig Ae/Be stars

Astronomy & Astrophysics 535 (2011): A99 reproduced with permission from Astronomy & AstrophysicsContext. The scarcity of accretion rate estimates and accretion tracers available for Herbig Ae/Be (HAeBe) stars contrasts with the extensive studies for lower mass objects. Aims. This work aims to derive accretion rates from the UV Balmer excess for a sample of 38 HAeBe stars. We look for possible empirical correlations with the strength of the Hα, [Oi]6300, and Brγ emission lines. Methods. Shock modelling within the context of magnetospheric accretion (MA) was applied to each star. We obtained the accretion rates from the excess in the Balmer discontinuity, derived from mean values of multi-epoch Johnson’s UB photometry. The accretion rates were related to both mean Hα luminosities, Hα 10% widths, and [Oi]6300 luminosities from simultaneous spectra, and to Brγ luminosities from the literature. Results. The typical -median- mass accretion rate is 2 × 10−7 M_ yr−1 in our sample, 36% of the stars showing values ≤10−7 M_ yr−1, 35% between 10−7 and 10−6, and 29% > 10−6 M_ yr−1. The model fails to reproduce the large Balmer excesses shown by the four hottest stars (T∗ > 12 000 K). When accretion is related to the stellar masses and luminosities (1 ≤ M∗/M_ ≤ 6; 2 ≤ L∗/L_ ≤ 103), we derive ˙Macc ∝ M5 ∗ and Lacc ∝ L1.2 ∗ , with scatter. Empirical calibrations relating the accretion and the Hα, [Oi]6300, and Brγ luminosities are provided. The slopes in our expressions are slightly shallower than those for lower mass stars, but the difference is within the uncertainties, except for the [Oi]6300 line. The Hα 10% width is uncorrelated with ˙Macc, unlike for the lower mass regime. The mean Hα width shows higher values as the projected rotational velocities of HAe stars increase, which agrees with MA. The accretion rate variations in the sample are typically lower than 0.5 dex on timescales of days to months. Our data suggest that the changes in the Balmer excess are uncorrelated to the simultaneous changes of the line luminosities. Conclusions. The Balmer excesses and Hα line widths of HAe stars can be interpreted within the context of MA, which is not the case for several HBes. The steep trend relating Macc and M∗ can be explained from the mass-age distribution characterizing HAeBe stars. The line luminosities used for low-mass objects are also valid to estimate typical accretion rates for the intermediate-mass regime under similar empirical expressions. However, we suggest that several of these calibrations are driven by the stellar luminosityC. Eiroa, I. Mendigutía, and B. Montesinos are partially supported by grant AYA-2008 01727. I. Mendigutía is grateful for financial support from the Space Telescope Science Institute during a two-month sta

The RMS survey: a census of massive YSO multiplicity in the KK-band

Close to 100 per cent of massive stars are thought to be in binary systems. The multiplicity of massive stars seems to be intrinsically linked to their formation and evolution, and Massive Young Stellar Objects are key in observing this early stage of star formation. We have surveyed three samples totalling hundreds of MYSOs ($>8M_\odot$) across the Galaxy from the RMS catalogue, using UKIDSS and VVV point source data, and UKIRT $K-$band imaging to probe separations between 0.8-9 arcsec (approx 1000-100,000 au). We have used statistical methods to determine the binary statistics of the samples, and we find binary fractions of $64\pm 4$ per cent for the UKIDSS sample, $53\pm 4$ per cent for the VVV sample, and $49\pm 8$ per cent for the RMS imaging sample. Also we use the $J-$ and $K-$band magnitudes as a proxy for the companion mass, and a significant fraction of the detected systems have estimated mass ratios greater than 0.5, suggesting a deviation from the capture formation scenario which would be aligned with random IMF sampling. Finally, we find that YSOs located in the outer Galaxy have a higher binary fraction than those in the inner Galaxy. This is likely due to a lower stellar background density than observed towards the inner Galaxy, resulting in higher probabilities for visual binaries to be physical companions. It does indicate a binary fraction in the probed separation range of close to 100 per cent without the need to consider selection biases.Comment: 14 pages, 9 figures, accepted to MNRA

Clustering properties of intermediate and high-mass Young Stellar Objects

We have selected 337 intermediate and high-mass YSOs ($1.5$ to $20$ M$_{\odot}$) well-characterised with spectroscopy. By means of the clustering algorithm HDBSCAN, we study their clustering and association properties in the Gaia DR3 catalogue as a function of stellar mass. We find that the lower mass YSOs ($1.5-4$ M$_{\odot}$) have clustering rates of $55-60\%$ in Gaia astrometric space, a percentage similar to the one found in the T Tauri regime. However, intermediate-mass YSOs in the range $4-10$ M$_{\odot}$ show a decreasing clustering rate with stellar mass, down to $27\%$. We find tentative evidence suggesting that massive YSOs ($>10$ M$_{\odot}$) often appear $-$yet not always$-$ clustered. We put forward the idea that most massive YSOs form via a mechanism that demands many low-mass stars around them. However, intermediate-mass YSOs form in a classical core-collapse T Tauri way, yet they do not appear often in the clusters around massive YSOs. We also find that intermediate and high-mass YSOs become less clustered with decreasing disk emission and accretion rate. This points towards an evolution with time. For those sources that appear clustered, no major correlation is found between their stellar properties and the cluster sizes, number of cluster members, cluster densities, or distance to cluster centres. In doing this analysis, we report the identification of 55 new clusters. We present tabulated all the derived cluster parameters for the considered intermediate and high-mass YSOs.Comment: Accepted for publication in The Astronomical Journal on August 18th, 2023. Table 1 and the new clusters can be provided upon reques

Probing the circumstellar structure of Herbig Ae/Be stars

We present Halpha spectropolarimetry observations of a sample of 23 Herbig Ae/Be stars. A change in the linear polarisation across Halpha is detected in a large fraction of the objects, which indicates that the regions around Herbig stars are flattened (disc-like) on small scales. A second outcome of our study is that the spectropolarimetric signatures for the Ae stars differ from those of the Herbig Be stars, with characteristics changing from depolarisation across Halpha in the Herbig Be stars, to line polarisations in the Ae group. The frequency of depolarisations detected in the Herbig Be stars (7/12) is particularly interesting as, by analogy to classical Be stars, it may be the best evidence to date that the higher mass Herbig stars are surrounded by flattened structures. For the Herbig Ae stars, 9 out of 11 show a line polarisation effect that can be understood in terms of a compact Halpha emission that is itself polarised by a rotating disc-like circumstellar medium. The spectropolarimetric difference between the Herbig Be and Ae stars may be the first indication that there is a transition in the Hertzsprung-Russell Diagram from magnetic accretion at spectral type A to disc accretion at spectral type B. Alternatively, the interior polarised line emission apparent in the Ae stars may be masked in the Herbig Be stars due to their higher levels of Halpha emission.Comment: 14 pages, MNRAS accepte

The circumstellar disk, envelope, and bi-polar outflow of the Massive Young Stellar Object W33A

The Young Stellar Object (YSO) W33A is one of the best known examples of a massive star still in the process of forming. Here we present Gemini North ALTAIR/NIFS laser-guide star adaptive-optics assisted K-band integral-field spectroscopy of W33A and its inner reflection nebula. In our data we make the first detections of a rotationally-flattened outer envelope and fast bi-polar jet of a massive YSO at near-infrared wavelengths. The predominant spectral features observed are Br-gamma, H_2, and a combination of emission and absorption from CO gas. We perform a 3-D spectro-astrometric analysis of the line emission, the first study of its kind. We find that the object's Br-gamma emission reveals evidence for a fast bi-polar jet on sub-milliarcsecond scales, which is aligned with the larger-scale outflow. The hybrid CO features can be explained as a combination of hot CO emission arising in a disk close to the central star, while cold CO absorption originates in the cooler outer envelope. Kinematic analysis of these features reveals that both structures are rotating, and consistent with being aligned perpendicularly to both the ionised jet and the large-scale outflow. Assuming Keplerian rotation, we find that the circumstellar disk orbits a central mass of >10Msun, while the outer envelope encloses a mass of ~15Msun. Our results suggest a scenario of a central star accreting material from a circumstellar disk at the centre of a cool extended rotating torus, while driving a fast bi-polar wind. These results therefore provide strong supporting evidence for the hypothesis that the formation mechanism for high-mass stars is qualitatively similar to that of low-mass stars.Comment: 13 pages, 18 figs. Accepted for publication in MNRA

The RMS Survey: Critical Tests of Accretion Models for the Formation of Massive Stars

There is currently no accepted theoretical framework for the formation of the most massive stars, and the manner in which protostars continue to accrete and grow in mass beyond \sim10Msun is still a controversial topic. In this study we use several prescriptions of stellar accretion and a description of the Galactic gas distribution to simulate the luminosities and spatial distribution of massive protostellar population of the Galaxy. We then compare the observables of each simulation to the results of the Red MSX Source (RMS) survey, a recently compiled database of massive young stellar objects. We find that the observations are best matched by accretion rates which increase as the protostar grows in mass, such as those predicted by the turbulent core and competitive accretion (i.e. Bondi-Hoyle) models. These 'accelerating accretion' models provide very good qualitative and quantitative fits to the data, though we are unable to distinguish between these two models on our simulations alone. We rule out models with accretion rates which are constant with time, and those which are initially very high and which fall away with time, as these produce results which are quantitatively and/or qualitatively incompatible with the observations. To simultaneously match the low- and high-luminosity YSO distribution we require the inclusion of a 'swollen-star' pre-main-sequence phase, the length of which is well-described by the Kelvin-Helmholz timescale. Our results suggest that the lifetime of the YSO phase is \sim 10^5yrs, whereas the compact Hii-region phase lasts between \sim 2 - 4 \times 10^5yrs depending on the final mass of the star. Finally, the absolute numbers of YSOs are best matched by a globally averaged star-formation rate for the Galaxy of 1.5-2Msun/yr.Comment: 22 pages, 24 figures. Accepted for publication in MNRA

Near-infrared line spectropolarimetry of hot massive stars

The definitive version is available at www.blackwell-synergy.com. Copyright Blackwell Publishing --DOI : 10.1111/j.1365-2966.2005.09598.xIn order to study the inner parts of the circumstellar material around optically faint, infrared bright objects, we present the first medium-resolution spectropolarimetric data taken in the near-infrared. In this paper we discuss Pa line data of GL 490, a well-known embedded massive young stellar object, and of MWC 349A and MWC 342, two optically faint stars that are proposed to be in the pre-main sequence phase of evolution. As a check on the method, the classical Be star Tau, known to display line polarization changes at optical wavelengths, was observed as well. Three of our targets show a “line effect” across Pa . For Tau and MWC 349A this line effect is due to depolarisation by a circumstellar electron-scattering disk. In both cases, the position angle of the polarisation is consistent with that of the larger scale disks imaged at other wavelengths, validating infrared spectropolarimetry as a means to detect flattening on small scales. The tentative detection of a rotation in the polarization position angle at Pa in the embedded massive young stellar object GL 490 suggests the presence of a small scale rotating accretion disk with an inner hole – similar to those recently discovered at optical wavelengths in Herbig Ae and T Tauri stars.Peer reviewe