An analysis of spectra in the Red Rectangle nebula

This paper presents an analysis of a series of spectra in the Red Rectangle nebula. Only the reddest part of the spectra can safely be attributed to light from the nebula, and indicates Rayleigh scattering by the gas, in conformity with the large angles of scattering involved and the proximity of the star. In the blue, light from HD44179, refracted or scattered in the atmosphere, dominates the spectra. This paper questions the reliability of ground-based observations of extended objects in the blue.Comment: 25 figure

Evolution of dust and ice features around FU Orionis objects

(abridged) We present spectroscopy data for a sample of 14 FUors and 2 TTauri stars observed with the Spitzer Space Telescope or with the Infrared Space Observatory (ISO). Based on the appearance of the 10 micron silicate feature we define 2 categories of FUors. Objects showing the silicate feature in absorption (Category 1) are still embedded in a dusty and icy envelope. The shape of the 10 micron silicate absorption bands is compared to typical dust compositions of the interstellar medium and found to be in general agreement. Only one object (RNO 1B) appears to be too rich in amorphous pyroxene dust, but a superposed emission feature can explain the observed shape. We derive optical depths and extinction values from the silicate band and additional ice bands at 6.0, 6.8 and 15.2 micron. In particular the analysis of the CO_2 ice band at 15.2 micron allows us to search for evidence for ice processing and constrains whether the absorbing material is physically linked to the central object or in the foreground. For objects showing the silicate feature in emission (Category 2), we argue that the emission comes from the surface layer of accretion disks. Analyzing the dust composition reveals that significant grain growth has already taken place within the accretion disks, but no clear indications for crystallization are present. We discuss how these observational results can be explained in the picture of a young, and highly active accretion disk. Finally, a framework is proposed as to how the two categories of FUors can be understood in a general paradigm of the evolution of young, low-mass stars. Only one object (Parsamian 21) shows PAH emission features. Their shapes, however, are often seen toward evolved stars and we question the object's status as a FUor and discuss other possible classifications.Comment: accepted for publication in ApJ; 63 pages preprint style including 8 tables and 24 figure

Will the starless cores in Chamaeleon I and III turn prestellar?

The nearby Chamaeleon molecular cloud complex is a good laboratory to study the process of low-mass star formation since it consists of three clouds with very different properties. Cha III does not show any sign of star formation, while star formation has been very active in Cha I and may already be finishing. Our goal is to determine whether star formation can proceed in Cha III, and to compare the results to our recent survey of Cha I. We used the Large APEX Bolometer Array (LABOCA) to map Cha III in dust continuum emission at 870 micron. 29 sources are extracted from the map, all of them being starless. The starless cores are found down to a visual extinction of 1.9 mag, in marked contrast with other molecular clouds, including Cha I. Apart from this difference, the Cha III starless cores share very similar properties with those found in Cha I. At most two sources have a mass larger than the critical Bonnor-Ebert mass, which suggests that the fraction of prestellar cores is very low, even lower than in Cha I. Only the most massive sources are candidate prestellar cores, in agreement with the correlation found earlier in the Pipe nebula. The mass distribution of the 85 starless cores of Cha I and III that are not candidate prestellar cores is consistent with a single power law down to the 90% completeness limit, with an exponent close to the Salpeter value. A fraction of the starless cores in Cha I and III may still grow in mass and become gravitationally unstable. Based on predictions of numerical simulations of turbulent molecular clouds, we estimate that at most 50% and 20% of the starless cores of Cha I and III, respectively, may form stars. The LABOCA survey reveals that Cha III, and Cha I to some extent too, is a prime target to study the formation of prestellar cores, and thus the onset of star formation. (abridged).Comment: Accepted for publication in A&A. 22 pages, 16 figures, 4 tables. A version with high-resolution figures is available on request to the first autho

The Spectral Energy Distribution of HH30 IRS: Constraining The Circumstellar Dust Size Distribution

We present spectral energy distribution (SED) models for the edge-on classical T Tauri star HH30 IRS that indicate dust grains have grown to larger than 50 microns within its circumstellar disk. The disk geometry and inclination are known from previous modeling of multiwavelength Hubble Space Telescope images and we use the SED to constrain the dust size distribution. Model spectra are shown for different circumstellar dust models: a standard ISM mixture and larger grain models. As compared to ISM grains, the larger dust grain models have a shallower wavelength dependent opacity. Models with the larger dust grains provide a good match to the currently available data, but mid and far-IR observations are required to more tightly constrain the dust size distribution. The accretion luminosity in our models is L_acc<0.2 L_star corresponding to an accretion rate of 4E-9M_sun/yr. Dust size distributions that are simple power-law extensions (i.e., no exponential cutoff) yield acceptable fits to the optical/near-IR but too much emission at mm wavelengths and require larger disk masses. Such a simple size distribution would not be expected in an environment such as the disk of HH30 IRS, particularly over such a large range in grain sizes. However, its ability to adequately characterize the grain populations may be determined from more complete observational sampling of the SED in the mid to far-IR.Comment: ApJ Accepte

Infrared Signatures of Protoplanetary Disk Evolution

We investigate the observational signatures of a straightforward evolutionary scenario for protoplanetary disks in which the disk mass of small (50 micron) particles decreases homologously with time, but the disk structure and stellar parameters do not change. Our goal is to identify optimal infrared spectral indicators of the existence of disks, their structure, and mass evolution that may be tested with the upcoming SIRTF mission. We present simulated spectral energy distributions and colors over a wide range of masses. The SED is most sensitive to disk mass in the far-IR and longer wavelengths, which is already known from millimeter and radio observations. As the disk mass decreases, the excess emission of the disk over the stellar photosphere diminishes more rapidly at the longest rather than at short wavelengths. At near-infrared wavelengths, the disk remains optically thick to stellar radiation over a wide range of disk mass, resulting in a slower decline in the SED in this spectral regime. Therefore, near-IR excesses (K-L) provide a robust means of detecting disks in star clusters down to 1E-7 solar masses, while the far-IR excess probes the disk mass. Reducing the disk mass results in a clear progression in color-color diagrams with low mass disks displaying the bluest colors. We interpret color-color diagrams for Taurus-Auriga sources in the context of decreasing disk mass.Comment: ApJ Accepte

On the Evolutionary Status of Class I Stars and Herbig-Haro Energy Sources in Taurus-Auriga

[abridged] We present high resolution optical spectra of stars in Taurus-Auriga whose circumstellar environment suggests that they are less evolved than optically revealed T Tauri stars. Many of the stars are seen only via scattered light. These spectra are used to search for differences between stars which power Herbig-Haro flows and stars which do not, and to reassess the evolutionary state of so-called protostars (Class I stars) relative to optically revealed T Tauri stars (Class II stars). The stellar mass distribution of Class I stars is similar to that of Class II stars and includes 3 Class I brown dwarfs. Class I stars in Taurus are slowly rotating; the angular momentum of a young star appears to dissipate prior to the optically revealed T Tauri phase. The mass accretion rates of Class I stars are surprisingly indistinguishable from those of Class II stars; they do not have accretion dominated luminosities. We confirm previous results that find larger forbidden-line emission associated with Class I stars than Class II stars. We suggest that this is caused by an orientation bias that allows a more direct view of the somewhat extended forbidden emission line regions than the obscured stellar photospheres, rather than because of larger mass outflow rates. Overall, the similar masses, luminosities, rotation rates, mass accretion rates, mass outflow rates, and millimeter flux densities of Class I and Class II stars are best explained by a scenario in which most Class I stars are no longer in the main accretion phase and are older than traditionally assumed. Similarly, although stars which power Herbig-Haro flows appear to have larger mass outflow rates, their stellar and circumstellar properties are generally indistinguishable from those of stars that do not power these flows.Comment: 84 pages, including 21 figures; accepted for publication in Ap

Modern optical astronomy: technology and impact of interferometry

The present `state of the art' and the path to future progress in high spatial resolution imaging interferometry is reviewed. The review begins with a treatment of the fundamentals of stellar optical interferometry, the origin, properties, optical effects of turbulence in the Earth's atmosphere, the passive methods that are applied on a single telescope to overcome atmospheric image degradation such as speckle interferometry, and various other techniques. These topics include differential speckle interferometry, speckle spectroscopy and polarimetry, phase diversity, wavefront shearing interferometry, phase-closure methods, dark speckle imaging, as well as the limitations imposed by the detectors on the performance of speckle imaging. A brief account is given of the technological innovation of adaptive-optics (AO) to compensate such atmospheric effects on the image in real time. A major advancement involves the transition from single-aperture to the dilute-aperture interferometry using multiple telescopes. Therefore, the review deals with recent developments involving ground-based, and space-based optical arrays. Emphasis is placed on the problems specific to delay-lines, beam recombination, polarization, dispersion, fringe-tracking, bootstrapping, coherencing and cophasing, and recovery of the visibility functions. The role of AO in enhancing visibilities is also discussed. The applications of interferometry, such as imaging, astrometry, and nulling are described. The mathematical intricacies of the various `post-detection' image-processing techniques are examined critically. The review concludes with a discussion of the astrophysical importance and the perspectives of interferometry.Comment: 65 pages LaTeX file including 23 figures. Reviews of Modern Physics, 2002, to appear in April issu

B[e] Stars with Warm Dust: Revealing the Nature of Unclassified B[e] Stars and Expanding the Family

Until recently, unclassified B[e] stars represented half of the entire B[e] group. Our study of these objects with strong emission-line spectra and IRAS fluxes, decreasing toward longer wavelengths, resulted in a suggestion that they currently form dust in their envelopes. The objects have been tentatively called B[e] stars with warm dust (B[e]WD). Their luminosity range (?3 orders of magnitude) is much larger compared to previous suggestions that dust formation occurs only near very luminous hot stars. A significant fraction of B[e]WD are recognized or suspected binaries. The group has been expanded with both previously detected hot emission-line stars with IR fluxes, typical for confirmed B[e]WD, and new candidates, found in recent all-sky surveys. Currently the number of B[e]WD members and candidates is ?60 with an opportunity to find more in existing stellar catalogs. Main observational and physical properties of B[e]WD and their envelopes are summarized. Our results on newly found group members are presented. Partially based on observations obtained at the Canada-France-Hawaii Telescope (CFHT).Fil: Miroshnichenko, A. S.. University of North Carolina at Greensboro; Estados UnidosFil: Bernabei, S.. Istituto Nazionale di Astrofisica; ItaliaFil: Bjorkman, K. S.. University Of Toledo (utoledo); Estados UnidosFil: Chentsov, E. L.. Russian Academy of Sciences; RusiaFil: Klochkova, V. G.. Russian Academy of Sciences; RusiaFil: Gray, R. O.. Appalachian State University; Estados UnidosFil: Levato, H.. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Complejo Astronómico "El Leoncito". Universidad Nacional de Córdoba. Complejo Astronómico "El Leoncito". Universidad Nacional de la Plata. Complejo Astronómico "El Leoncito". Universidad Nacional de San Juan. Complejo Astronómico "El Leoncito"; ArgentinaFil: Grosso, Monica Gladys. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Instituto de Ciencias Astronómicas, de la Tierra y del Espacio. Universidad Nacional de San Juan. Instituto de Ciencias Astronómicas, de la Tierra y del Espacio; ArgentinaFil: Hinkle, K. H.. National Optical Astronomy Observatory; Estados UnidosFil: Kuratov, K. S.. Fesenkov Astrophysical Institute; KazajistánFil: Kusakin, A. V.. Universitetskij pr; RusiaFil: García Lario, P.. European Space Astronomy Centre; EspañaFil: Perea Calderón, J. V.. European Space Astronomy Centre; EspañaFil: Polcaro, V. F.. Istituto di Astrofisica Spaziale e Fisica Cosmica; ItaliaFil: Viotti, R. F.. Istituto di Astrofisica Spaziale e Fisica Cosmica; ItaliaFil: Norci, L.. Dublin City University; IrlandaFil: Manset, N.. Canada France Hawaii Telescope; Estados UnidosFil: Men’shchikov, A. B.. Saint Mary’s University; CanadáFil: Rudy, R. J.. The Aerospace Corporation; Estados UnidosFil: Lynch, D. K.. The Aerospace Corporation; Estados UnidosFil: Venturini, C. C.. The Aerospace Corporation; Estados UnidosFil: Mazuk, S.. The Aerospace Corporation; Estados UnidosFil: Puetter, R. C.. University of California; Estados UnidosFil: Perry, R. B.. National Aeronautics and Space Administration; Estados UnidosFil: Gandet, T. L.. Lizard Hollow Observatory; Estados Unido

An independent distance estimate to CW Leo

CW Leo has been observed six times between October 2009 and June 2012 with the SPIRE instrument on board the Herschel satellite. Variability has been detected in the flux emitted by the central star with a period of 639 \pm 4 days, in good agreement with determinations in the literature. Variability is also detected in the bow shock around CW Leo that had previously been detected in the ultraviolet and Herschel PACS/SPIRE data. Although difficult to prove directly, our working hypothesis is that this variability is directly related to that of the central star. In this case, fitting a sine curve with the period fixed to 639 days results in a time-lag in the variability between bow shock and the central star of 402 \pm 37 days. The orientation of the bow shock relative to the plane of the sky is unknown (but see below). For an inclination angle of zero degrees, the observed time-lag translates into a distance to CW Leo of 130 \pm 13 pc, and for non-zero inclination angles the distance is smaller. Fitting the shape of the bow shock with an analytical model (Wilkin 1996), the effect of the inclination angle on the distance may be estimated. Making the additional assumption that the relative peculiar velocity between the interstellar medium (ISM) and CW Leo is determined entirely by the star space velocity with respect to the local standard of rest (i.e. a stationary ISM), the inclination angle is found to be (-33.3 \pm 0.8) degrees based on the observed proper motion and radial velocity. Using the Wilkin model, our current best estimate of the distance to CW Leo is 123 \pm 14 pc. For a distance of 123 pc, we derive a mean luminosity of 7790 \pm 150 Lsol (internal error).Comment: Accepted A&A Letter

A census of dense cores in the Aquila cloud complex: SPIRE/PACS observations from the <i>Herschel</i> Gould Belt survey

We present and discuss the results of the $Herschel$ Gould Belt survey (HGBS) observations in an ~11 $deg^2$ area of the Aquila molecular cloud complex at $d$ ~ 260 pc, imaged with the SPIRE and PACS photometric cameras in parallel mode from $70\mu m$ to $500\mu m$. Using the multi-scale, multi-wavelength source extraction algorithm $getsources$, we identify a complete sample of starless dense cores and embedded (Class 0-I) protostars in this region, and analyze their global properties and spatial distributions. We find a total of 651 starless cores, ~60% ± 10% of which are gravitationally bound prestellar cores, and they will likely form stars in the future. We also detect 58 protostellar cores. The core mass function (CMF) derived for the large population of prestellar cores is very similar in shape to the stellar initial mass function (IMF), confirming earlier findings on a much stronger statistical basis and supporting the view that there is a close physical link between the stellar IMF and the prestellar CMF. The global shift in mass scale observed between the CMF and the IMF is consistent with a typical star formation efficiency of ~40% at the level of an individual core. By comparing the numbers of starless cores in various density bins to the number of young stellar objects (YSOs), we estimate that the lifetime of prestellar cores is ~1 Myr, which is typically ~4 times longer than the core free-fall time, and that it decreases with average core density. We find a strong correlation between the spatial distribution of prestellar cores and the densest filaments observed in the Aquila complex. About 90% of the $Herschel$-identified prestellar cores are located above a background column density corresponding to $A_V$ ~ 7, and ~75% of them lie within filamentary structures with supercritical masses per unit length ≳16 $M_{\odot}/pc$. These findings support a picture wherein the cores making up the peak of the CMF (and probably responsible for the base of the IMF) result primarily from the gravitational fragmentation of marginally supercritical filaments. Given that filaments appear to dominate the mass budget of dense gas at $A_V> 7$, our findings also suggest that the physics of prestellar core formation within filaments is responsible for a characteristic “efficiency” $SFR/M_{dense}$ ~$5^{+2}_{-2}x 10^{-8}yr^{-1}$ for the star formation process in dense gas