Correlations between the accretion luminosity and emission line luminosities (L_acc and L_line) of pre-main sequence (PMS) stars have been published for many different spectral lines, which are used to estimate accretion rates. Despite the origin of
those correlations is unknown, this could be attributed to direct or indirect physical relations between the emission line formation and the accretion mechanism. This work shows that all (near-UV/optical/near-IR) L_acc-L_line correlations are the result of the fact that the accretion luminosity and the stellar luminosity (L_star) are correlated, and are not necessarily related with the physical origin of the line. Synthetic and observational data are used to illustrate how the L_acc-L_line correlations depend on the L_acc-L_star relationship. We conclude that because PMS stars show the L_acc-L_star correlation immediately implies that L_acc also correlates with the luminosity of all emission lines, for which the L_acc-L_line correlations alone do not prove any physical connection with accretion but can only be used with practical purposes to roughly estimate accretion rates. When looking for correlations with possible physical meaning, we suggest that L_acc/L_star and L_line/L_star should be used instead of L_acc and L_line. Finally, the finding that L_acc has a steeper dependence on L_star for T-Tauri stars than for intermediate-mass Herbig Ae/Be stars is also discussed. That is explained from the magnetospheric accretion scenario and the different photospheric properties in the near-UV.
The circumstellar structure on 100 AU scales of the massive young stellar object W33A is probed using the VLTI and the MIDI instrument. N-band visibilities on 4 baselines are presented which are inconsistent with a spherically symmetric geometry. The
visibility spectra and SED are simultaneously compared to 2D axi-symmetric dust radiative transfer models with a geometry including a rotationally flattened envelope and outflow cavities. We assume an O7.5 ZAMS star as the central source, consistent with the observed bolometric luminosity. The observations are also compared to models with and without (dusty and gaseous) accretion disks. A satisfactory model is constructed which reproduces the visibility spectra for each (u,v) point. It fits the silicate absorption, the mid-IR slope, the far-infrared peak, and the (sub)mm of the SED. It produces a 350 micron morphology consistent with observations. The 10 micron emission on 100 AU scales is dominated by the irradiated walls of the cavity sculpted by the outflow. The visibilities rule out the presence of dust disks with total (gas and dust) masses more than 0.01 Msun. However, optically thick accretion disks, interior to the dust sublimation radius, are allowed to accrete at rates equalling the envelopes mass infall rate (up to 10^(-3) Msun/yr) without substantially affecting the visibilities due to the extinction by the extremely massive envelope of W33A.
We present diffraction limited (0.6) 24.5micron Subaru/COMICS images of the red supergiant mu Cep. We report the detection of a circumstellar nebula, that was not detected at shorter wavelengths. It extends to a radius of at least 6 in the thermal in
frared. On these angular scales, the nebula is roughly spherical, in contrast, it displays a pronounced asymmetric morphology closer in. We simultaneously model the azimuthally averaged intensity profile of the nebula and the observed spectral energy distribution using spherical dust radiative transfer models. The models indicate a constant mass-loss process over the past 1000 years, for mass-loss rates a few times 10^(-7) Msun/yr. This work supports the idea that at least part of the asymmetries in shells of evolved massive stars and supernovae may be due to the mass-loss process in the red supergiant phase.
We present interferometric and single-dish mid-infrared observations of a sample of massive young stellar objects (BN-type objects), using VLTI-MIDI (10 micron) and Subaru-COMICS (24.5 micron). We discuss the regions S140, Mon R2, M8E-IR, and W33A. T
he observations probe the inner regions of the dusty envelope at scales of 50 milli arcsecond and 0.6 arcsec (100-1000 AU), respectively. Simultaneous model fits to spectral energy distributions and spatial data are achieved using self-consistent spherical envelope modelling. We conclude that those MYSO envelopes that are best described by a spherical geometry, the commensurate density distribution is a powerlaw with index -1.0. Such a powerlaw is predicted if the envelope is supported by turbulence on the 100-1000AU scales probed with MIDI and COMICS, but the role of rotation at these spatial scales need testing.
The paper investigates the milli-arcsecond scale structure of the present-day mass-loss of the post-Red Supergiant IRC+10420. We use three telescopes of the VLT Interferometer in combination with the AMBER near-infrared beam combiner to measure spect
rally dispersed correlated fluxes in the K-band around the Br gamma transition. The resulting visibilities are compared to the predicted visibilities of emission structures with various simple models in order to infer the size of the observed emission region. The Br gamma line is resolved by VLTI+AMBER on all three baselines, with the maximum projected baseline extending 69 meter and a P.A. ranging between 10 and 30 degrees. A differential phase between line and continuum is detected on the longest baseline. The Br gamma emission region is found to have a diameter of 3.3 milli-arcseconds (FWHM), when compared to a Gaussian intensity distribution. A uniform disk and a ring-like intensity distribution do not fit the line visibilities. Comparing the AMBER equivalent width of Br gamma with measurements from various epochs, we find that the stellar photosphere contributes about 60% of the total continuum light at 2.2 micron. The remaining 40% continuum emission is found on scales larger than the 66mas AMBER field of view. Using simple arguments, and assuming optically thick line emission, we find that the line emitting region is elongated. We briefly discuss the possibilities whether such a structure is due to a bi-polar flow or a circumstellar disk. (Abridged).
