CO and dust emission at millimeter wavelengths are independent tracers of cold interstellar matter, which have seldom been compared on the scale of giant molecular clouds (GMCs) in other galaxies. In ...this study, and for the first time for the Large Magellanic Cloud (LMC), we compute the molecular cloud masses from the millimeter emission of the dust and compare them with the masses derived from their CO luminosity and virial theorem. We present CO (J = 1–0 and J = 2–1) and 1.2 mm continuum observations of the N11 star-forming region in the LMC obtained with the SEST telescope and the SIMBA bolometer, respectively. We use the CO data to identify individual molecular clouds and measure their physical properties (CO luminosity, size, line width, and virial masses). The correlations between the properties of the N11 clouds agree with those found in earlier studies in the LMC that sample a larger set of clouds and a wider range of cloud masses. For the N11 molecular clouds, we compare the masses estimated from the CO luminosity (XCO LCO), the virial theorem (Mvir) and the millimeter dust luminosity (ℒ1.2 mm(dust)). The measured ratios LCO/Mvir and ℒ1.2 mm(dust)/Mvir constrain the XCO and \hbox{$\mathcal{K}_{\rm dust}$}𝒦dust (dust emissivity at 1.2 mm per unit gas mass) parameters as a function of the virial parameter αvir. The comparison between the different mass estimates yields a XCO-factor of 8.8 ± 3.5 × 1020 cm-2 (K km s-1) -1 αvir-1 and a \hbox{$\mathcal{K}_{\rm dust}$}𝒦dust parameter of 1.5 ± 0.5 × 10-3 cm2 g-1 αvir. We compare our N11 results with a similar analysis for molecular clouds in the Gould Belt in the solar neighborhood. We do not find a large discrepancy in N11 between the dust millimeter and virial masses as reported in earlier studies of molecular clouds in the Small Magellanic Cloud. The ratio between ℒ1.2 mm and Mvir in N11 is half of what is measured for Gould Belt clouds, which can be accounted for by a factor of two lower gas-to-dust mass ratio, as the difference in gas metallicities. If the two samples have similar αvir values, this result implies that their dust far-IR properties are also similar.
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Aims. We present the first NIR spectro-interferometry of the LBV η Carinae. The observations were performed with the AMBER instrument of the ESO Very Large Telescope Interferometer (VLTI) using ...baselines from 42 to 89 m. The aim of this work is to study the wavelength dependence of η Car's optically thick wind region with a high spatial resolution of 5 mas (11 AU) and high spectral resolution. Methods. The observations were carried out with three 8.2 m Unit Telescopes in the K-band. The raw data are spectrally dispersed interferograms obtained with spectral resolutions of 1500 (MR-K mode) and 12 000 (HR-K mode). The MR-K observations were performed in the wavelength range around both the $\ion{He}{i}$ 2.059 μm and the Brγ 2.166 μm emission lines, the HR-K observations only in the Brγ line region. Results. The spectrally dispersed AMBER interferograms allow the investigation of the wavelength dependence of the visibility, differential phase, and closure phase of η Car. In the K-band continuum, a diameter of $4.0\pm0.2$ mas (Gaussian FWHM, fit range 28–89 m baseline length) was measured for η Car's optically thick wind region. If we fit Hillier et al. (2001, ApJ, 553, 837) model visibilities to the observed AMBER visibilities, we obtain 50% encircled-energy diameters of 4.2, 6.5 and 9.6 mas in the 2.17$\,\mu$m continuum, the $\ion{He}{i}$, and the Brγ emission lines, respectively. In the continuum near the Brγ line, an elongation along a position angle of $120\degr\pm15\degr$ was found, consistent with previous VINCI/VLTI measurements by van Boekel et al. (2003, A&A, 410, L37). We compare the measured visibilities with predictions of the radiative transfer model of Hillier et al. (2001), finding good agreement. Furthermore, we discuss the detectability of the hypothetical hot binary companion. For the interpretation of the non-zero differential and closure phases measured within the Brγ line, we present a simple geometric model of an inclined, latitude-dependent wind zone. Our observations support theoretical models of anisotropic winds from fast-rotating, luminous hot stars with enhanced high-velocity mass loss near the polar regions.
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Context.CO observations have been the best way so far to trace molecular gas in external galaxies, but in low metallicity environments the gas mass deduced could be largely underestimated due to ...enhanced photodissociation of the CO molecule. Large envelopes of H2 could therefore be missed by CO observations. Aims.At present, the kinematic information of CO data cubes are used to estimate virial masses and trace the total mass of the molecular clouds. Millimeter dust emission can also be used as a dense gas tracer and could unveil H2 envelopes lacking CO. These different tracers must be compared in different environments. Methods.This study compares virial masses to masses deduced from millimeter emission, in two GMC samples: the local molecular clouds in our Galaxy (104-105 M$_\odot$), and their equivalents in the Small Magellanic Cloud (SMC), one of the nearest low metallicity dwarf galaxies. Results. In our Galaxy, mass estimates deduced from millimeter (FIRAS) emission are consistent with masses deduced from gamma ray analysis and therefore trace the total mass of the clouds. Virial masses are systematically larger (twice on average) than mass estimates from millimeter dust emission. This difference decreases toward high masses and has been reported in previous studies. This is not the case for SMC giant molecular clouds: molecular cloud masses deduced from SIMBA millimeter observations are systematically higher (twice on average for conservative values of the dust to gas ratio and dust emissivity) than the virial masses from SEST CO observations. The observed excess cannot be accounted for by any plausible change of dust properties. Taking a general form for the virial theorem, we show that a magnetic field strength of ~15 μG in SMC clouds could provide additional support for the clouds and explain the difference observed. Conclusions.We conclude that masses of SMC molecular clouds have so far been underestimated. Magnetic pressure may contribute significantly to their support.
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Aims.We report on spectrally dispersed interferometric AMBER/VLTI observations of the recurrent nova RS Oph five days after the discovery of its outburst on 2006 Feb. 12. Methods.Using three ...baselines ranging from 44 to 86 m, and a spectral resolution of $\lambda/\delta\lambda=1500$, we measured the extension of the milliarcsecond-scale emission in the K band continuum and in the Brγ and He I $2.06~\mu$m lines, allowing us to get an insight into the kinematics of the line forming regions. The continuum visibilities were interpreted by fitting simple geometric models consisting of uniform and Gaussian ellipses, ring and binary models. The visibilities and differential phases in the Brγ line were interpreted using skewed ring models aiming to perform a limited parametric reconstruction of the extension and kinematics of the line forming region. Results.The limited uv coverage does not allow discrimination between filled models (uniform or Gaussian ellipses) and rings. Binary models are discarded because the measured closure phase in the continuum is close to zero. The visibilities in the lines are at a low level compared to their nearby continuum, consistent with a more extended line forming region for He I $2.06~\mu$m than Brγ. The ellipse models for the continuum and for the lines are highly flattened ($b/a\sim0.6$) and share the same position angle (${\rm PA}\sim140^{\circ}$). Their typical Gaussian extensions are $3.1\times1.9~{\rm mas}$, $4.9\times2.9~{\rm mas}$ and $6.3\times3.6~{\rm mas}$ for the continuum, Brγ and He I $2.06~\mu$m lines, respectively. Two radial velocity fields are apparent in the Brγ line: a slow expanding ring-like structure ($v_{\rm rad}\leq1800~{\rm km\,s}^{-1}$), and a fast structure extended in the E-W direction ($v_{\rm rad}\sim 2500{-}3000~{\rm km\,s}^{-1}$), a direction that coincides with the jet-like structure seen in the radio. These results confirm the basic fireball model, contrary to the conclusions of other interferometric observations conducted by Monnier et al. (2006a, ApJ, 647, L127).
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Context. We present the first high spatial-resolution monitoring of the dust-forming nova V1280 Sco, performed with the Very Large Telescope Interferometer (VLTI). Aims. These observations promise to ...improve the distance determination of such events and constrain the mechanisms leading to very efficient dust formation under the harsh physical conditions encountered in novae ejecta. Methods. Spectra and visibilities were regularly acquired between the onset of dust formation, 23 days after discovery (or 11 days after maximum), and day 145, using the beam-combiner instruments AMBER (near-IR) and MIDI (mid-IR). These interferometric observations were complemented by near-infrared data from the 1.2 m Mt. Abu Infrared Observatory, India. The observations are initially interpreted in terms of simple uniform models; however more complex models, probably involving a second shell, are required to explain data acquired following $t=110$ d after outburst. This behavior is in accordance with the light curve of V1280 Sco, which exhibits a secondary peak at about $t=106$ d, followed by a new, steep decline, suggesting a new dust-forming event. Spherical dust shell models generated with the DUSTY code are used to investigate the parameters of the main dust shell. Results. Using uniform disk models, these observations allow us to determine an apparent linear expansion rate for the dust shell of $0.35 \pm 0.03$ mas day-1 and the approximate ejection time of the matter in which dust formed of tejec = 10.5 ± 7 d, i.e. close to the maximum brightness. This information, combined with the expansion velocity of 500 ± 100 km s-1, implies a distance estimate of 1.6 ± 0.4 kpc. The sparse uv coverage does not enable deviations from spherical symmetry to be clearly discerned. The dust envelope parameters were determined. The dust mass generated was typically 2–8 $\times$ 10-9 $M_\odot$ day-1, with a probable peak in production at about 20 days after the detection of dust and another peak shortly after $t=110$ d, when the amount of dust in the shell was estimated as 2.2 $\times$ 10-7 $M_\odot$. Considering that the dust-forming event lasted at least 200–250 d, the mass of the ejected material is likely to have exceeded 10-4 $M_\odot$. The conditions for the formation of multiple shells of dust are also discussed.
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Aims.We investigate the origin of the ${\rm Br}\gamma$ emission of the Herbig Ae star HD 104237 on Astronomical Unit (AU) scales. Methods.Using AMBER/VLTI at a spectral resolution $\mathcal{R}=1500$ ...we spatially resolve the emission in both the ${\rm Br}\gamma$ line and the adjacent continuum. Results.The visibility does not vary between the continuum and the ${\rm Br}\gamma$ line, even though the line is strongly detected in the spectrum, with a peak intensity 35% above the continuum. This demonstrates that the line and continuum emission have similar size scales. We assume that the K-band continuum excess originates in a “puffed-up” inner rim of the circumstellar disk, and discuss the likely origin of ${\rm Br}\gamma$. Conclusions.We conclude that this emission most likely arises from a compact disk wind, launched from a region 0.2–0.5 AU from the star, with a spatial extent similar to that of the near infrared continuum emission region, i.e., very close to the inner rim location.
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Radio observations of different molecular species in comet C/1995 O1(Hale-Bopp) have been carried out regularly since August 1995. We present an analysis of carbon monoxide spectra observed at SEST. ...A detailed picture of the nature of the post-perihelion outgassing at large heliocentric distances is shown. The data starts at 2.9 AU, with active water sublimation, and stretch outside 10 AU, where CO-driven activity is still detected. The activity is studied using a coma model which separates the outgassing into two different sources, one insolation-dependent source, peaking at the subsolar point, and one isotropically outgassing source. Gas velocities and production rates of the two sources are derived for two different cases. In the first case the isotropic source is assumed to be inside the nucleus, and in the second case this source is assumed to be CO-rich material in the coma. Under both assumptions, the results indicate that the two component sources evolve independently. At heliocentric distances around 3 AU, acceleration of the CO molecules in the innermost region of the coma is required for the model to fit the observations. This acceleration must occur at a slower rate than that predicted by hydrodynamic simulations. From 6 to 10 AU, the derived parameters appear remarkably constant. The nuclear outgassing velocity is generally unchanged with heliocentric distance during this wide interval. Observations of molecules other than CO are included in the SEST dataset, and a comparison is made to the modelling results. HCN and CH3OH show signs of being produced in the same way as the CO up to a heliocentric distance of 6 AU.
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The young stellar object MWC 297 is an embedded B1.5Ve star exhibiting strong hydrogen emission lines and a strong near-infrared continuum excess. This object has been observed with the VLT ...interferometer equipped with the AMBER instrument during its first commissioning run. AMBER/VLTI is currently the only near infrared interferometer that can observe spectrally dispersed visibilities. MWC 297 has been spatially resolved in the continuum with a visibility of $0.50^{+0.08}_{-0.10}$ as well as in the Brγ emission line where the visibility decreases to $0.33\pm0.06$. This change in the visibility with wavelength can be interpreted by the presence of an optically thick disk responsible for the visibility in the continuum and of a stellar wind traced by the Brγ emission line and whose apparent size is 40% larger. We validate this interpretation by building a model of the stellar environment that combines a geometrically thin, optically thick accretion disk model consisting of gas and dust, and a latitude-dependent stellar wind outflowing above the disk surface. The continuum emission and visibilities obtained from this model are fully consistent with the interferometric AMBER data. They agree also with existing optical, near-infrared spectra and other broad-band near-infrared interferometric visibilities. We also reproduce the shape of the visibilities in the Brγ line as well as the profile of this line obtained at an higher spectral resolution with the VLT/ISAAC spectrograph, and those of the Hα and Hβ lines. The disk and wind models yield a consistent inclination of the system of approximately 20°. A picture emerges in which MWC 297 is surrounded by an equatorial flat disk that is possibly still accreting and an outflowing wind that has a much higher velocity in the polar region than at the equator. The AMBER/VLTI unique capability of measuring spectral visibilities therefore allows us for the first time to compare the apparent geometry of a wind with the disk structure in a young stellar system.
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Aims.We study the geometry and kinematics of the circumstellar environment of the Be star κ CMa in the Brγ emission line and its nearby continuum. Methods.We use the AMBER/VLTI instrument operating ...in the K band, which provides a spatial resolution of about 6 mas with a spectral resolution of 1500, to study the kinematics within the disk and to infer its rotation law. To obtain more kinematical constraints we also use a high spectral resolution Paβ line profile obtain in December 2005 at the Observatorio do Pico do Dios, Brazil and we compile $V/R$ line profile variations and spectral energy distribution data points from the literature. Results.Using differential visibilities and differential phases across the Brγ line we detect an asymmetry in the disk. Moreover, we found that κ CMa seems difficult to fit within the classical scenario for Be stars, illustrated recently by α Arae observations, i.e. a fast rotating B star close to its breakup velocity surrounded by a Keplerian circumstellar disk with an enhanced polar wind. We discuss the possibility that κ CMa is a critical rotator with a Keplerian rotating disk and examine whether if the detected asymmetry can be interpreted within the “one-armed” viscous disk framework.
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