Background derivation and image flattening: getimages

Modern high-resolution images obtained with space observatories display extremely strong intensity variations across images on all spatial scales. Source extraction in such images with methods based on global thresholding may bring unacceptably large numbers of spurious sources in bright areas while failing to detect sources in low-background or low-noise areas. It would be highly beneficial to subtract background and equalize the levels of small-scale fluctuations in the images before extracting sources or filaments. This paper describes getimages, a new method of background derivation and image flattening. It is based on median filtering with sliding windows that correspond to a range of spatial scales from the observational beam size up to a maximum structure width $X_{\lambda}$. The latter is a single free parameter of getimages that can be evaluated manually from the observed image $\mathcal{I}_{\lambda}$. The median filtering algorithm provides a background image $\tilde{\mathcal{B}}_{\lambda}$ for structures of all widths below $X_{\lambda}$. The same median filtering procedure applied to an image of standard deviations $\mathcal{D}_{\lambda}$ derived from a background-subtracted image $\tilde{\mathcal{S}}_{\lambda}$ results in a flattening image $\tilde{\mathcal{F}}_{\lambda}$. Finally, a flattened detection image $\mathcal{I}_{{\lambda}\mathrm{D}}{\,=\,}\tilde{\mathcal{S}}_{\lambda}{/}\tilde{\mathcal{F}}_{\lambda}$ is computed, whose standard deviations are uniform outside sources and filaments. Detecting sources in such greatly simplified images results in much cleaner extractions that are more complete and reliable. As a bonus, getimages reduces various observational and map-making artifacts and equalizes noise levels between independent tiles of mosaicked images.Comment: 14 pages, 11 figures (main text + 3 appendices), accepted by Astronomy & Astrophysics; fixed Metadata abstract (typesetting

Inaccuracies and biases of the Gaussian size deconvolution for extracted sources and filaments

A simple Gaussian size deconvolution method is routinely used to remove the blur of observed images caused by insufficient angular resolutions of existing telescopes, thereby to estimate the physical sizes of extracted sources and filaments. The size deconvolution method is expected to work when the structures, as well as the telescope beams, have Gaussian shapes. This study employs model images of the spherical and cylindrical objects with Gaussian and power-law shapes, representing the dense cores and filaments. The images are convolved to a wide range of angular resolutions to probe various degrees of resolvedness of the models. Simplified flat, convex, and concave backgrounds are added to the images, then planar backgrounds across the footprints of the structures are subtracted and sizes are measured and deconvolved. When background subtraction is inaccurate, the structures acquire profoundly non-Gaussian profiles. The deconvolved half maximum sizes can be strongly under- or overestimated, by factors of up to ~20 when the structures are unresolved or partially resolved. For resolved structures, the errors are within a factor of ~2, although for some power-law models show the factors of up to ~6. The size deconvolution method cannot be applied to unresolved structures, it can be used only for the Gaussian-like structures, including the critical Bonnor-Ebert spheres, when they are at least partially resolved. The method must be considered inapplicable for the power-law structures with shallow profiles. This work also reveals subtle properties of convolution for different geometries. When convolved with different kernels, spherical objects and cylindrical filaments with identical profiles obtain different widths and shapes. A filament, imaged by the telescope with a non-Gaussian PSF, could appear substantially shallower than the structure is in reality, even when it is resolved.Comment: 20 pages, 17 figures, 5 tables, accepted by Astronomy & Astrophysic

Properties of the close binary and circumbinary torus of the Red Rectangle

New diffraction-limited speckle images of the Red Rectangle in the wavelength range 2.1--3.3 microns with angular resolutions of 44--68 mas and previous speckle images at 0.7--2.2 microns revealed well-resolved bright bipolar outflow lobes and long X-shaped spikes originating deep inside the outflow cavities. This set of high-resolution images stimulated us to reanalyze all infrared observations of the Red Rectangle using our two-dimensional radiative transfer code. The new detailed modeling, together with estimates of the interstellar extinction in the direction of the Red Rectangle enabled us to more accurately determine one of the key parameters, the distance D=710 pc with model uncertainties of 70 pc, which is twice as far as the commonly used estimate of 330 pc. The central binary is surrounded by a compact, massive (M=1.2 Msun), very dense dusty torus with hydrogen densities reaching n_H=2.5x10^12 cm^-3 (dust-to-gas mass ratio rho_d/rho~0.01). The bright component of the spectroscopic binary HD 44179 is a post-AGB star with mass M*=0.57 Msun, luminosity L*=6000 Lsun, and effective temperature T*=7750 K. Based on the orbital elements of the binary, we identify its invisible component with a helium white dwarf with Mwd~0.35 Msun, Lwd~100 Lsun, and Twd~6x10^4 K. The hot white dwarf ionizes the low-density bipolar outflow cavities inside the dense torus, producing a small HII region observed at radio wavelengths. We propose an evolutionary scenario for the formation of the Red Rectangle nebula, in which the binary initially had 2.3 and 1.9 Msun components at a separation of 130 Rsun. The nebula was formed in the ejection of a common envelope after Roche lobe overflow by the present post-AGB star.Comment: 20 pages, 10 figures, accepted by Astronomy and Astrophysics, also available at http://www.mpifr-bonn.mpg.de/div/ir-interferometry/publications.htm

The formation of active protoclusters in the Aquila Rift: A millimeter continuum view

Abridged -- We present an analysis of the Aquila Rift complex which addresses the questions of the star formation rate (SFR), star formation efficiency (SFE) and typical lifetime of the Class 0 protostellar phase in two nearby cluster-forming clumps: the Serpens South and W40 protoclusters. We carried out a 1.2 mm dust continuum mapping of the Aquila Rift complex with the MAMBO bolometer array on the IRAM 30m telescope. We perform a systematic source extraction in our millimeter continuum map. Based on complementary data from the Herschel Gould Belt survey and Spitzer maps, we characterize the SEDs of the 77 mm continuum sources detected with MAMBO and estimate their evolutionary stages. Taking advantage of the comprehensive dataset available for the Serpens South region, spanning wavelengths from 2 microns to 1.2 mm, we estimate the numbers of young stellar objects (YSOs) at different evolutionary stages and find a ratio of Class 0 to Class I protostars N(0)/N(I)=0.19-0.27. This low ratio supports a scenario of relatively fast accretion at the beginning of the protostellar phase, and leads to a Class 0 lifetime of ~4-9x10^{4} yr. We also show that both the Serpens South and W40 protoclusters are characterized by large fractions of protostars and high SFRs ~20-50 Msun.Myr^{-1}pc^{-2}, in agreement with the idea that these two nearby clumps are active sites of clustered star formation currently undergoing bursts of star formation, and have the potential ability to form bound star clusters. While the formation of these two protoclusters is likely to have been initiated in a very different manner, the resulting protostellar populations are observed to be very similar. This suggests that after the onset of gravitational collapse, the detailed manner in which the collapse has been initiated does not affect much the ability of a clump to form stars.Comment: 11 pages, 5 figures. Abstract has been shortened. Accepted for publication in A&A. Final version including corrections in section 4.1.2. Table 1 available upon request, contact the author

AZEuS: An Adaptive Zone Eulerian Scheme for Computational MHD

A new adaptive mesh refinement (AMR) version of the ZEUS-3D astrophysical magnetohydrodynamical (MHD) fluid code, AZEuS, is described. The AMR module in AZEuS has been completely adapted to the staggered mesh that characterises the ZEUS family of codes, on which scalar quantities are zone-centred and vector components are face-centred. In addition, for applications using static grids, it is necessary to use higher-order interpolations for prolongation to minimise the errors caused by waves crossing from a grid of one resolution to another. Finally, solutions to test problems in 1-, 2-, and 3-dimensions in both Cartesian and spherical coordinates are presented.Comment: 52 pages, 17 figures; Accepted for publication in ApJ

Properties of the dense cores and filamentary structures in the Vela C molecular cloud

The initial and boundary conditions of the Galactic star formation in molecular clouds are not well understood. In an effort to shed new light on this long-standing problem, we measured properties of dense cores and filamentary structures in the Vela C molecular cloud, observed with Herschel. We applied the getsf extraction method to separate the components of sources and filaments from each other and their backgrounds, before detecting, measuring, and cataloging the structures. The cores and filamentary structures constitute 40% of the total mass of Vela C, most of the material is in the low-density molecular background cloud. We selected 570 reliable cores, of which 149 are the protostellar cores and 421 are the starless cores. Almost 78% of the starless cores were identified with the gravitationally bound prestellar cores. The exponent of the CMF (alpha = 1.35) is identical to that of the Salpeter IMF. We selected 68 filaments with at least one side that appeared not blended with adjacent structures. The filament widths are in the range of 0.15 pc to 0.63 pc, and have a median value of W = 0.3(0.11) pc. The surface densities of filaments are well correlated with their contrasts and linear densities. Within uncertainties of the filament instability criterion, many filaments may well be both supercritical and subcritical. A large fraction of filaments may definitely be considered supercritical, in which are found 94 prestellar cores, 83 protostellar cores, and only 1 unbound starless core. Taking into account the uncertainties, the supercritical filaments contain only prestellar and protostellar cores. Our findings support the idea that there exists a direct relationship between the CMF and IMF and that filaments play a key role in the formation of prestellar cores, which is consistent with the previous Herschel results.Comment: 15 pages, 12 figures, 3 tables, accepted for publication in A&

Low-Mass Binary Induced Outflows from Asymptotic Giant Branch Stars

A significant fraction of planetary nebulae (PNe) and proto-planetary nebulae (PPNe) exhibit aspherical, axisymmetric structures, many of which are highly collimated. The origin of these structures is not entirely understood, however recent evidence suggests that many observed PNe harbor binary systems, which may play a role in their shaping. In an effort to understand how binaries may produce such asymmetries, we study the effect of low-mass (< 0.3 M_sun) companions (planets, brown dwarfs and low-mass main sequence stars) embedded into the envelope of a 3.0 M_sun star during three epochs of its evolution (Red Giant Branch, Asymptotic Giant Branch (AGB), interpulse AGB). We find that common envelope evolution can lead to three qualitatively different consequences: (i) direct ejection of envelope material resulting in a predominately equatorial outflow, (ii) spin-up of the envelope resulting in the possibility of powering an explosive dynamo driven jet and (iii) tidal shredding of the companion into a disc which facilitates a disc driven jet. We study how these features depend on the secondary's mass and discuss observational consequences.Comment: 24 pages, 6 figures, submitted to MNRA

Preliminary Results of the Herschel Gould Belt Survey in the Orion B Complex

As a preliminary result of the Herschel Gould Belt survey (André et al. 2010) in the Orion B cloud complex we find a clear connection between the locations of the detected prestellar cores and the column density values. We find that the vast majority of the gravitationally bound prestellar cores are detected above a high column density of about 6-7 × 1021 cm-2 (A V ˜ 6-7). This is in very good agreement with dense core formation thresholds found in other regions. For Orion B, a similar limit appears both in the distribution of background column density values of the prestellar cores, and in the column density PDF of the region. Within our core formation scenario, the found threshold can be translated as the column density above which the filaments become gravitationally unstable and fragment into cores

Strong dependence of the physical properties of cores on spatial resolution in observations and simulations

International audienceThe angular resolution of a telescope is the primary observational parameter, along with the detector sensitivity in defining the quality of the observed images and of the subsequent scientific exploitation of the data. During the last decade in star formation research, many studies have targeted low- and high-mass star formation regions located at different distances, with different telescopes having specific angular resolution capabilities. However, no dedicated studies of the spatial resolution effects on the derived sizes and masses of the sources extracted from the observed images have been published. We present a systematic investigation of the angular resolution effects, with special attention being paid to the derived masses of sources as well as the shape of the resulting source mass functions (SMFs) and to their comparison with the initial stellar mass function. For our study, we chose two star-forming regions observed with Herschel, NGC 6334 and Aquila distant of 1750 and 460 pc respectively, and three (magneto)-hydrodynamical simulations, virtually positioned at the same distances as the observed regions. We built surface density maps with different angular resolutions by convolving the surface density images of the five regions to a set of four resolutions differing by a factor of two (9, 18, 36, and 72′′), which allowed us to cover spatial resolutions from 0.6 down to 0.02 pc. Then we detected and measured sources in each of the images at each resolution using getsf and we analysed the derived masses and sizes of the extracted sources. We find that the number of sources does not converge from 0.6 to ≳0.05 pc. It increases by about two when the angular resolution increases with a similar factor, which confirms that these large sources are cluster-forming clumps. Below 0.05 pc, the number of source still increases by about 1.3 when the angular resolution increases by two, suggesting that we are close to, but not yet at, convergence. In this regime of physical scales, we find that the measured sizes and masses of sources linearly depend on the angular resolution with no sign of convergence to a resolution-independent value, implying that these sources cannot be assimilated to isolated prestellar cores. The corresponding SMF peak also shifts with angular resolution, while the slope of the high-mass tail of the SMFs remains almost invariant. We propose that these angular resolution effects could be caused by the underestimated background of the unresolved sources observed against the sloping, hill-like backgrounds of the molecular clouds. If prestellar cores physically distinct from their background exist in cluster-forming molecular clouds, we conclude that their mass must be lower than reported so far in the literature. We discuss various implications for the studies of star formation: the problem of determining the mass reservoirs involved in the star-formation process; the inapplicability of the Gaussian beam deconvolution to infer source sizes; and the impossibility to determine the efficiency of the mass conversion from the cores to the stars. Our approach constitutes a simple convergence test to determine whether an observation is affected by angular resolution