Interferometric Identification of a Pre-Brown Dwarf André, Philippe; Ward-Thompson, Derek; Greaves, Jane
Science (American Association for the Advancement of Science),
07/2012, Volume:
337, Issue:
6090
Journal Article
Peer reviewed
Open access
It is not known whether brown dwarfs stellar-like objects with masses less than the hydrogen-burning limit, 0.075 solar mass (MSymbol) are formed in the same way as solar-type stars or by some other ...process. Here we report the clear-cut identification of a self-gravitating condensation of gas and dust with a mass in the brown-dwarf regime, made through millimeter interferometric observations. The level of thermal millimeter continuum emission detected from this object indicates a mass -0.02 to 0.03 MSymbol, whereas the small radius, < 460 astronomical units, and narrow spectral lines imply a dynamical mass of 0.015 to 0.02 MSymbol. The identification of such a pre-brown dwarf core supports models according to which brown dwarfs are formed in the same manner as hydrogen-burning stars.
We determine the magnetic field strength in the OMC 1 region of the Orion A filament via a new implementation of the Chandrasekhar-Fermi method using observations performed as part of the James Clerk ...Maxwell Telescope (JCMT) B-Fields In Star-forming Region Observations (BISTRO) survey with the POL-2 instrument. We combine BISTRO data with archival SCUBA-2 and HARP observations to find a plane-of-sky magnetic field strength in OMC 1 of mG, where mG represents a predominantly systematic uncertainty. We develop a new method for measuring angular dispersion, analogous to unsharp masking. We find a magnetic energy density of J m−3 in OMC 1, comparable both to the gravitational potential energy density of OMC 1 (∼10−7 J m−3) and to the energy density in the Orion BN/KL outflow (∼10−7 J m−3). We find that neither the Alfvén velocity in OMC 1 nor the velocity of the super-Alfvénic outflow ejecta is sufficiently large for the BN/KL outflow to have caused large-scale distortion of the local magnetic field in the ∼500 yr lifetime of the outflow. Hence, we propose that the hourglass field morphology in OMC 1 is caused by the distortion of a primordial cylindrically symmetric magnetic field by the gravitational fragmentation of the filament and/or the gravitational interaction of the BN/KL and S clumps. We find that OMC 1 is currently in or near magnetically supported equilibrium, and that the current large-scale morphology of the BN/KL outflow is regulated by the geometry of the magnetic field in OMC 1, and not vice versa.
We present the first high-resolution, submillimeter-wavelength polarimetric observations of-and thus direct observations of the magnetic field morphology within-the dense gas of the Pillars of ...Creation in M16. These 850 m observations, taken as part of the B-Fields in Star-forming Region Observations Survey (BISTRO) using the POL-2 polarimeter on the Submillimeter Common-User Bolometer Array 2 (SCUBA-2) camera on the James Clerk Maxwell Telescope (JCMT), show that the magnetic field runs along the length of the Pillars, perpendicular to and decoupled from the field in the surrounding photoionized cloud. Using the Chandrasekhar-Fermi method we estimate a plane-of-sky magnetic field strength of 170-320 G in the Pillars, consistent with their having been formed through the compression of gas with initially weak magnetization. The observed magnetic field strength and morphology suggests that the magnetic field may be slowing the Pillars' evolution into cometary globules. We thus hypothesize that the evolution and lifetime of the Pillars may be strongly influenced by the strength of the coupling of their magnetic field to that of their parent photoionized cloud-i.e., that the Pillars' longevity results from magnetic support.
The dependence of the polarization fraction p on total intensity I in polarized submillimeter emission measurements is typically parameterized as p ∝ I− ( ≤ 1) and used to infer dust grain alignment ...efficiency in star-forming regions, with an index = 1 indicating near-total lack of alignment of grains with the magnetic field. In this work, we demonstrate that the non-Gaussian noise characteristics of the polarization fraction may produce apparent measurements of ∼ 1 even in data with significant signal-to-noise in Stokes Q, U, and I emission, and so with robust measurements of polarization angle. We present a simple model demonstrating this behavior and propose a criterion by which well-characterized measurements of the polarization fraction may be identified. We demonstrate that where our model is applicable, can be recovered by fitting the p-I relationship with the mean of the Rice distribution without statistical debiasing of the polarization fraction. We apply our model to JCMT BISTRO Survey POL-2 850 m observations of three clumps in the Ophiuchus molecular cloud, finding that in the externally illuminated Oph A region, 0.34, while in the more isolated Oph B and C, despite their differing star formation histories, ∼ 0.6-0.7. Our results thus suggest that dust grain alignment in dense gas is more strongly influenced by the incident interstellar radiation field than by star formation history. We further find that grains may remain aligned with the magnetic field at significantly higher gas densities than has previously been believed, thus allowing investigation of magnetic field properties within star-forming clumps and cores.
Guiding the reader through all the stages that lead to the formation of a star such as our Sun, this advanced textbook provides students with a complete overview of star formation. It examines the ...underlying physical processes that govern the evolution from a molecular cloud core to a main-sequence star, and focuses on the formation of solar-mass stars. Each chapter combines theory and observation, helping readers to connect with and understand the theory behind star formation. Beginning with an explanation of the interstellar medium and molecular clouds as sites of star formation, subsequent chapters address the building of typical stars and the formation of high-mass stars, concluding with a discussion of the by-products and consequences of star formation. This is a unique, self-contained text with sufficient background information for self-study, and is ideal for students and professional researchers alike.
We present 850 m polarization observations of the L1689 molecular cloud, part of the nearby Ophiuchus molecular cloud complex, taken with the POL-2 polarimeter on the James Clerk Maxwell Telescope ...(JCMT). We observe three regions of L1689: the clump L1689N which houses the IRAS 16293-2433 protostellar system, the starless clump SMM-16, and the starless core L1689B. We use the Davis-Chandrasekhar-Fermi method to estimate plane-of-sky field strengths of 366 55 G in L1689N, 284 34 G in SMM-16, and 72 33 G in L1689B, for our fiducial value of dust opacity. These values indicate that all three regions are likely to be magnetically transcritical with sub-Alfvénic turbulence. In all three regions, the inferred mean magnetic field direction is approximately perpendicular to the local filament direction identified in Herschel Space Telescope observations. The core-scale field morphologies for L1689N and L1689B are consistent with the cloud-scale field morphology measured by the Planck Space Observatory, suggesting that material can flow freely from large to small scales for these sources. Based on these magnetic field measurements, we posit that accretion from the cloud onto L1689N and L1689B may be magnetically regulated. However, in SMM-16, the clump-scale field is nearly perpendicular to the field seen on cloud scales by Planck, suggesting that it may be unable to efficiently accrete further material from its surroundings.
We estimate the dust temperatures of the clumps in the ρ Oph main cloud taking into account the 3D geometry of the region, and external heating from the interstellar radiation field and from HD ...147879, a nearby luminous B2V star, which is believed to dominate the radiation field in the region. We find that the regions where pre-stellar cores are observed (i.e. at optical visual extinctions >7 mag) are colder than ∼10 –11 K. 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 pre-stellar cores in the ρ Oph main cloud from millimetre 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 α∼ 1.5 to a slope α∼ 2.5 has moved from ∼0.5 to ∼1 M⊙. 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 (∼0.2 M⊙), but the CMF may flatten at around ∼0.4 M⊙. We generalize our results to the pre-stellar cores in Taurus and in Orion. In Taurus, the ambient radiation field heating the pre-stellar cores is believed to be weaker than that in ρ 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 103 times stronger than the standard interstellar radiation field. Based on this assumption, we estimate that the dust temperatures of the pre-stellar cores in Orion are around ∼20 –30 K.
ABSTRACT We present results for Vela C obtained during the 2012 flight of the Balloon-borne Large Aperture Submillimeter Telescope for Polarimetry. We mapped polarized intensity across almost the ...entire extent of this giant molecular cloud, in bands centered at 250, 350, and 500 m. In this initial paper, we show our 500 m data smoothed to a resolution of 2 5 (approximately 0.5 pc). We show that the mean level of the fractional polarization p and most of its spatial variations can be accounted for using an empirical three-parameter power-law fit, , where N is the hydrogen column density and S is the polarization-angle dispersion on 0.5 pc scales. The decrease of p with increasing S is expected because changes in the magnetic field direction within the cloud volume sampled by each measurement will lead to cancellation of polarization signals. The decrease of p with increasing N might be caused by the same effect, if magnetic field disorder increases for high column density sightlines. Alternatively, the intrinsic polarization efficiency of the dust grain population might be lower for material along higher density sightlines. We find no significant correlation between N and S. Comparison of observed submillimeter polarization maps with synthetic polarization maps derived from numerical simulations provides a promising method for testing star formation theories. Realistic simulations should allow for the possibility of variable intrinsic polarization efficiency. The measured levels of correlation among p, N, and S provide points of comparison between observations and simulations.
We present Herschel SPIRE and PACS maps of the Cepheus Flare clouds L1157, L1172, L1228, L1241, and L1251, observed by the Herschel Gould Belt Survey of nearby star-forming molecular clouds. Through ...modified blackbody fits to the SPIRE and PACS data, we determine typical cloud column densities of (0.5-1.0) × 1021 cm−2 and typical cloud temperatures of 14-15 K. Using the getsources identification algorithm, we extract 832 dense cores from the SPIRE and PACS data at 160-500 m. From placement in a mass versus size diagram, we consider 303 to be candidate prestellar cores, and 178 of these to be "robust" prestellar cores. From an independent extraction of sources at 70 m, we consider 25 of the 832 dense cores to be protostellar. The distribution of background column densities coincident with candidate prestellar cores peaks at (2-4) × 1021 cm−2. About half of the candidate prestellar cores in Cepheus may have formed as a result of the widespread fragmentation expected to occur within filaments of "transcritical" line mass. The lognormal robust prestellar core mass function (CMF) drawn from all five Cepheus clouds peaks at 0.56 M and has a width of ∼0.5 dex, similar to that of Aquila's CMF. Indeed, the width of Cepheus's aggregate CMF is similar to the stellar system initial mass function (IMF). The similarity of CMF widths in different clouds and the system IMF suggests a common, possibly turbulent origin for seeding the fluctuations that evolve into prestellar cores and stars.
PDF
