arXiv subjects

D. Stamatellos

Publications and source records attributed to D. Stamatellos.

31 records · Page 2Linked to original sources

arXiv2008

Simulating star formation in molecular cloud cores IV. The role of turbulence and thermodynamics

We perform SPH simulations of the collapse and fragmentation of low-mass cores having different initial levels of turbulence (alpha_turb=0.05,0.10,0.25). We use a new treatment of the energy equation which captures the transport of cooling radiation against opacity due to both dust and gas (including the effects of dust sublimation, molecules, and H^- ions). We also perform comparison simulations using a standard barotropic equation of state. We find that -- when compared with the barotropic equation of state -- our more realistic treatment of the energy equation results in more protostellar objects being formed, and a higher proportion of brown dwarfs; the multiplicity frequency is essentially unchanged, but the multiple systems tend to have shorter periods (by a factor ~3), higher eccentricities, and higher mass ratios. The reason for this is that small fragments are able to cool more effectively with the new treatment, as compared with the barotropic equation of state. We find that the process of fragmentation is often bimodal. The first protostar to form is usually, at the end, the most massive, i.e. the primary. However, frequently a disc-like structure subsequently forms round this primary, and then, once it has accumulated sufficient mass, quickly fragments to produce several secondaries. We believe that this delayed fragmentation of a disc-like structure is likely to be an important source of very low-mass hydrogen-burning stars and brown dwarfs.

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arXiv2007

SCUBA and Spitzer observations of the Taurus molecular cloud - pulling the bull's tail

We present continuum data from the Submillimetre Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT), and the Mid-Infrared Photometer for Spitzer (MIPS) on the Spitzer Space Telescope, at submillimetre and infrared wavelengths respectively. We study the Taurus molecular cloud 1 (TMC1), and in particular the region of the Taurus Molecular Ring (TMR). In the continuum data we see no real evidence for a ring, but rather we see one side of it only, appearing as a filament. We name the filament `the bull's tail'. The filament is seen in emission at 850, 450 and 160um, and in absorption at 70um. We compare the data with archive data from the Infra-Red Astronomical Satellite (IRAS) at 12, 25, 60, 100um, in which the filament is also seen in absorption. We find that the emission from the filament consists of two components: a narrow, cold (~8K), central core; and a broader, slightly warmer (~12K), shoulder of emission. We use a radiative transfer code to model the filament's appearance, either in emission or absorption, simultaneously at each of the different wavelengths. Our best fit model uses a Plummer-like density profile and a homogeneous interstellar dust grain population. Unlike previous work on a similar, but different filament in Taurus, we require no grain coagulation to explain our data.

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arXiv2007

Radiative transfer and the energy equation in SPH simulations of star formation

We introduce and test a new and highly efficient method for treating the thermal and radiative effects influencing the energy equation in SPH simulations of star formation. The method uses the density, temperature and gravitational potential of each particle to estimate a mean optical depth, which then regulates the particle's heating and cooling. The method captures -- at minimal computational cost -- the effects of (i) the rotational and vibrational degrees of freedom of H2, H2 dissociation, H0 ionisation, (ii) opacity changes due to ice mantle melting, sublimation of dust, molecular lines, H-, bound-free and free-free processes and electron scattering; (iv) external irradiation; and (v) thermal inertia. The new algorithm reproduces the results of previous authors and/or known analytic solutions. The computational cost is comparable to a standard SPH simulation with a simple barotropic equation of state. The method is easy to implement, can be applied to both particle- and grid-based codes, and handles optical depths 0<tau<10^{11}.

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arXiv2007

The James Clerk Maxwell Telescope Legacy Survey of Nearby Star-forming Regions in the Gould Belt

This paper describes a James Clerk Maxwell Telescope (JCMT) legacy survey that has been awarded roughly 500 hrs of observing time to be carried out from 2007 to 2009. In this survey we will map with SCUBA-2 (Submillimetre Common User Bolometer Array 2) almost all of the well-known low-mass and intermediate-mass star-forming regions within 0.5 kpc that are accessible from the JCMT. Most of these locations are associated with the Gould Belt. From these observations we will produce a flux-limited snapshot of star formation near the Sun, providing a legacy of images, as well as point-source and extended-source catalogues, over almost 700 square degrees of sky. The resulting images will yield the first catalogue of prestellar and protostellar sources selected by submillimetre continuum emission, and should increase the number of known sources by more than an order of magnitude. We will also obtain CO maps with the array receiver HARP (Heterodyne Array Receiver Programme), in three CO isotopologues, of a large typical sample of prestellar and protostellar sources. We will then map the brightest hundred sources with the SCUBA-2 polarimeter (POL-2), producing the first statistically significant set of polarization maps in the submillimetre. The images and source catalogues will be a powerful reference set for astronomers, providing a detailed legacy archive for future telescopes, including ALMA, Herschel and JWST.

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arXiv2007

The dust temperatures of the prestellar cores in the rho Oph main cloud and in other star forming regions: consequences for the core mass function

We estimate the dust temperatures of the clumps in the rho Oph main cloud taking into account the 3D geometry of the region, and external heating from the interstellar radiation field and from HD147879,a nearby luminous B2V star, which is believed to dominate the radiation field in the region. We find that the regions where prestellar cores are observed (i.e. at optical visual extinctions >7 mag) are colder than ~10-11K. These dust temperatures are smaller than those which previous studies of the same region have assumed. We use the new dust temperatures to estimate the masses of the prestellar cores in the rho Oph main cloud from mm observations, and we find core masses that are larger than previous estimates by a factor of ~2-3. This affects the core mass function (CMF) of the region; we find that the mass at which the core mass spectrum steepens from a slope alpha~1.5 to a slope alpha~2.5 has moved from ~0.5 Msun to ~1Msun. In contrast with the CMF in other star forming regions (e.g. Orion), there is no indication for a turnover down to the completeness limit (~0.2Msun), but the CMF may flatten at around ~0.4Msun. We generalize our results to the prestellar cores in Taurus and in Orion. In Taurus the ambient radiation field heating the prestellar cores is believed to be weaker than than that in rho Oph. Hence, the dust temperatures of the cores in Taurus are expected to be below ~ 10-11 K.In Orion the radiation field is believed to be 10^3 times stronger than the standard interstellar radiation field. Based on this assumption we estimate that the dust temperatures of the prestellar cores in Orion are around ~20-30K.

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arXiv2006

A VLA search for young protostars embedded in dense cores

Four dense cores, L1582A, L1689A, B133 and B68, classified as prestellar in terms of the absence of detectable NIR emission, are observed at radio wavelengths to investigate whether they nurture very young protostars. No definite young protostars were discovered in any of the four cores observed. A few radio sources were discovered close to the observed cores, but these are most likely extragalactic sources or YSOs unrelated to the cores observed. In L1582A we discovered a weak radio source near the centre of the core with radio characteristics and offset from the peak of the submillimeter emission similar to that of the newly discovered protostar in the core L1014, indicating a possible protostellar nature for this source. This needs to be confirmed with near- and/or mid-infrared observations (e.g. with Spitzer). Hence based on the current observations we are unable to confirm unequivocally that L1582A is starless. In L1689A a possible 4.5-sigma radio source was discovered at the centre of the core, but needs to be confirmed with future observations. In B133 a weak radio source, possibly a protostar, was discovered at the edge of the core on a local peak of the core submm emission, but no source was detected at the centre of the core. Thus, B133 is probably starless, but may have a protostar at its edge. In B68 no radio sources were discovered inside or at the edge of the core, and thus B68 is indeed starless. Four more radio sources with spectral indices characteristic of young protostars were discovered outside the cores but within the extended clouds in which these cores reside. Conclusions: We conclude that the number of cores misclassified as prestellar is probably very small and does not significantly alter the estimated lifetime of the prestellar phase.

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arXiv2006

The minimum mass for star formation, and the origin of binary brown dwarfs

Our first aim is to calculate the minimum mass for Primary Fragmentation in a variety of potential star-formation scenarios, i.e. (i) hierarchical fragmentation of a 3-D medium; (ii) one-shot, 2-D fragmentation of a shock-compressed layer; (iii) fragmentation of a circumstellar disc. Our second aim is to evaluate the role of H2 dissociation in facilitating Secondary Fragmentation and thereby producing close, low-mass binaries. Results: (i)For contemporary, local star formation, the minimum mass for Primary Fragmentation is in the range 0.001-0.004Msun, irrespective of the scenario considered. (ii)Circumstellar discs are only able to radiate fast enough to undergo Primary Fragmentation in their cool outer parts (R>100AU). Therefore brown dwarfs (BDs) should have difficulty forming by Primary Fragmentation at R<30AU, explaining the Brown Dwarf Desert.Conversely, Primary Fragmentation at R>100AU could be the source of brown dwarfs in wide orbits, and could explain why massive discs with Rd>100AU are rarely seen.(iii)H2 dissociation can lead to collapse and Secondary Fragmentation, thereby converting primary fragments into close, low-mass binaries, with semi-major axes a~5AU(Msystem/0.1Msun), in good agreement with observation; in this case, the minimum mass for Primary Fragmentation becomes a minimum system mass, rather than a minimum stellar mass.(iv)Any primary fragment can undergo Secondary Fragmentation, producing a close low-mass binary, provided only that the fragment is spinning. Secondary Fragmentation is therefore most likely in fragments formed in the outer parts of discs, and this could explain why a BD in a wide orbit about a Sun-like star has a greater likelihood of having a BD companion than a BD in the field -as seems to be observed.

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arXiv2005

Observational characteristics of dense cores with deeply embedded young protostars

Class 0 objects, which are thought to be the youngest protostars, are identified in terms of NIR or radio emission and/or the presence of molecular outflows. We present combined hydrodynamic and radiative transfer simulations of the collapse of a star-forming molecular core, which suggest two criteria for identifying dense cores with deeply embedded very young protostars that may not be observable in the NIR or radio with current telescopes. We find that cores with protostars are relatively warm (T>15K) with their SEDs peaking at wavelengths <170 micron, and they tend to appear circular.

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arXiv2005

How to identify the youngest protostars

We study the transition from a prestellar core to a Class 0 protostar, using SPH to simulate the dynamical evolution, and a Monte Carlo radiative transfer code to generate the SED and isophotal maps. For a prestellar core illuminated by the standard interstellar radiation field, the luminosity is low and the SED peaks at ~190 micron. Once a protostar has formed, the luminosity rises (due to a growing contribution from accretion onto the protostar) and the peak of the SED shifts to shorter wavelengths (~80-100 micron). However, by the end of the Class 0 phase, the accretion rate is falling, the luminosity has decreased, and the peak of the SED shifts back towards longer wavelengths (90-150 micron). In our simulations, the density of material around the protostar remains sufficiently high well into the Class 0 phase that the protostar only becomes visible in the NIR if it is displaced from the centre dynamically. Raw submm/mm maps of Class 0 protostars tend to be much more centrally condensed than those of prestellar cores. However, when convolved with a typical telescope beam, the difference in central concentration is less marked, although the Class 0 protostars appear more circular. Our results suggest that, if a core is deemed to be prestellar on the basis of having no associated IRAS source, no cm radio emission, and no outflow, but it has a circular appearance and an SED which peaks at wavelengths below ~170 micron, it may well contain a very young Class 0 protostar.

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arXiv2004

Dust temperatures in asymmetric prestellar cores

We present 2D Monte Carlo radiative transfer simulations of flattened prestellar cores. We argue the importance of observing prestellar cores near the peak of their emission spectra, and we point out observable characteristic features on isophotal maps of asymmetric cores at FIR wavelengths that are indicative of the cores' density and temperature structure. These features are on scales 1/5 to 1/3 of the overall core size, and so high resolution observations are needed to observe them. Finally, we discuss the importance of determining the temperatures of prestellar cores with high accuracy.

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arXiv2004

Radiative transfer models of non-spherical prestellar cores

We present 2D Monte Carlo radiative transfer simulations of prestellar cores. We consider two types of asymmetry: disk-like asymmetry, in which the core is denser towards the equatorial plane than towards the poles; and axial asymmetry, in which the core is denser towards the south pole than the north pole. We limit our treatment to cores with mild asymmetries, which are exposed directly to the interstellar radiation field or are embedded inside molecular clouds. The isophotal maps of a core depend strongly on the viewing angle. Maps at wavelengths longer than the peak of the SED (e.g. 850 micron) essentially trace the column-density. Thus, for instance, cores with disk-like asymmetry appear elongated when mapped at 850 micron from close to the equatorial plane. However, at wavelengths near the peak of the SED (e.g. 200 micron), the emissivity is more strongly dependent on the temperature, and therefore, at particular viewing angles, there are characteristic features which reflect a more complicated convolution of the density and temperature fields within the core. These characteristic features are on scales 1/5 to 1/3 of the overall core size, and so high resolution observations are needed to observe them. They are also weaker if the core is embedded in a molecular cloud (because the range of temperature within the core is then smaller), and so high sensitivity is needed to detect them. Herschel, to be launched in 2007, will in principle provide the necessary resolution and sensitivity at 170 to 250 micron.

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arXiv2003

Monte Carlo Radiative Transfer in Embedded Prestellar Cores

We implement a Monte Carlo radiative transfer method, that uses a large number of monochromatic luminosity packets to represent the radiation transported through a system. These packets are injected into the system and interact stochastically with it. We test our code against various benchmark calculations and determine the number of packets required to obtain accurate results under different circumstances. We then use this method to study cores that are directly exposed to the interstellar radiation field (non-embedded cores) and find similar results with previous studies. We also explore a large number of models of cores that are embedded in the centre of a molecular cloud. Our study indicates that the temperature profiles in embedded cores are less steep than those in non-embedded cores. Deeply embedded cores (ambient cloud with visual extinction larger than 15-25) are almost isothermal at around 7-8 K. The temperature inside cores surrounded by an ambient cloud of even moderate thickness (Av~5) is less than 12 K, which is lower than previous studies have assumed. Thus, previous mass calculations of embedded cores (for example in the rho Ophiuchi protocluster), based on mm continuum observations, may underestimate core masses by up to a factor of 2. Our study shows that the best wavelength region to observe embedded cores is between 400 and 500 microns, where the core is quite distinct from the background. We also predict that very sensitive observations (~1-3 MJy/sr) at 170-200 microns can be used to estimate how deeply a core is embedded in its parent molecular cloud. The upcoming HERSCHEL mission (ESA, 2007) will, in principle, be able to detect these features and test our models.

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arXiv2003

Radiative Transfer in Prestellar Cores: A Monte Carlo Approach

We use our Monte Carlo radiative transfer code to study non-embedded prestellar cores and cores that are embedded at the centre of a molecular cloud. Our study indicates that the temperature inside embedded cores is lower than in isolated non-embedded cores, and generally less than 12 K, even when the cores are surrounded by an ambient cloud of small visual extinction (Av~5). Our study shows that the best wavelength region to observe embedded cores is between 400 and 500 microns, where the core is quite distinct from the background. We also predict that very sensitive observations (~1-3 MJy/sr) at 170-200 microns can be used to estimate how deeply a core is embedded in its parent molecular cloud. Finally, we present preliminary results of asymmetric models of non-embedded cores.

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