These are the latest articles published in The Astrophysical Journal.

• Open/close The Small-scale Physical Structure and Fragmentation Difference of Two Embedded Intermediate-mass Protostars in Orion

  T. A. van Kempen et al. 2012 ApJ 751 137 Tag this article

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  Intermediate-mass (IM) protostars, the bridge between the very common solar-like protostars and the more massive, but rarer, O and B stars, can only be studied at high physical spatial resolutions in a handful of clouds. In this paper, we present and analyze the continuum results from an observing campaign at the Submillimeter Array (SMA) targeting two well-studied IM protostars in Orion, NGC 2071 and L1641 S3 MMS 1. The extended SMA (eSMA) probes structure at angular resolutions up to 0 2, revealing protostellar disks on scales of ~200 AU. Continuum flux measurements on these scales indicate that a significant amount of mass, a few tens of M _☉, is present. Envelope, stellar, and disk masses are derived using compact, extended, and eSMA configurations and compared against spectral energy distribution fitting models. We hypothesize that fragmentation into three components occurred within NGC 2071 at an early time, when the envelopes were less than 10% of their current masses, e.g., <0.5 M _☉. No fragmentation occurred for L1641 S3 MMS 1. For NGC 2071, evidence is given that the bulk of the envelope material currently around each source was accreted after the initial fragmentation. In addition, about 30% of the total core mass is not yet associated to one of the three sources. A global accretion model is favored and a potential accretion history of NGC 2071 is presented. It is shown that the relatively low level of fragmentation in NGC 2071 was stifled compared to the expected fragmentation from a Jeans argument. Similarly, the lack of fragmentation in L1641 S3 MMS 1 is likely due to similar arguments.

• Open/close A Young Stellar Cluster within the RCW41 H II Region: Deep NIR Photometry and Optical/NIR Polarimetry

  Fábio P. Santos et al. 2012 ApJ 751 138 Tag this article

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  The RCW41 star-forming region is embedded within the Vela Molecular Ridge, hosting a massive stellar cluster surrounded by a conspicuous H II region. Understanding the role of interstellar magnetic fields and studying the newborn stellar population is crucial to building a consistent picture of the physical processes acting on this kind of environment. We carried out a detailed study of the interstellar polarization toward RCW41 with data from an optical and near-infrared polarimetric survey. Additionally, deep near-infrared images from the 3.5 meter New Technology Telescope were used to study the photometric properties of the embedded young stellar cluster, revealing several YSO candidates. By using a set of pre-main-sequence isochrones, a mean cluster age in the range 2.5-5.0 million years was determined, and evidence of sequential star formation was revealed. An abrupt decrease in R-band polarization degree was noticed toward the central ionized area, probably due to low grain alignment efficiency caused by the turbulent environment and/or the weak intensity of magnetic fields. The distortion of magnetic field lines exhibits dual behavior, with the mean orientation outside the area approximately following the borders of the star-forming region and directed radially toward the cluster inside the ionized area, in agreement with simulations of expanding H II regions. The spectral dependence of polarization allowed a meaningful determination of the total-to-selective extinction ratio by fittings of the Serkowski relation. Furthermore, a large rotation of polarization angle as a function of wavelength was detected toward several embedded stars.

• Open/close Physical Properties of Lyα Emitters at z ~ 0.3 from UV-to-FIR Measurements

  I. Oteo et al. 2012 ApJ 751 139 Tag this article

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  The analysis of the physical properties of low-redshift Lyα emitters (LAEs) can provide clues in the study of their high-redshift analogs. At z ~ 0.3, LAEs are bright enough to be detected over almost the entire electromagnetic spectrum and it is possible to carry out a more precise and complete study than at higher redshifts. In this work, we examine the UV and IR emission, dust attenuation, star formation rate (SFR), and morphology of a sample of 23 GALEX-discovered star-forming LAEs at z ~ 0.3 with direct UV ( GALEX), optical (ACS), and FIR (PACS and MIPS) data. Using the same UV and IR limiting luminosities, we find that LAEs at z ~ 0.3 tend to be less dusty, have slightly higher total SFRs, have bluer UV continuum slopes, and are much smaller than other galaxies that do not exhibit Lyα emission in their spectrum (non-LAEs). These results suggest that at z ~ 0.3, Lyα photons tend to escape from small galaxies with low dust attenuation. Regarding their morphology, LAEs belong to Irr/merger classes, unlike non-LAEs. Size and morphology represent the most noticeable difference between LAEs and non-LAEs at z ~ 0.3. Furthermore, the comparison of our results with those obtained at higher redshifts indicates either that the Lyα technique picks up different kind of galaxies at different redshifts or that the physical properties of LAEs are evolving with redshift.

• Open/close The ELM Survey. IV. 24 White Dwarf Merger Systems

  Mukremin Kilic et al. 2012 ApJ 751 141 Tag this article

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  We present new radial velocity and X-ray observations of extremely low mass (ELM, ~0.2 M _☉) white dwarf (WD) candidates in the Sloan Digital Sky Survey Data Release 7 area. We identify seven new binary systems with 1-18 hr orbital periods. Five of the systems will merge due to gravitational wave radiation within 10 Gyr, bringing the total number of merger systems found in the ELM Survey to 24. The ELM Survey has now quintupled the known merger WD population. It has also discovered the eight shortest period detached binary WD systems currently known. We discuss the characteristics of the merger and non-merger systems observed in the ELM Survey, including their future evolution. About half of the systems have extreme mass ratios. These are the progenitors of the AM Canum Venaticorum systems and Type Ia supernovae. The remaining targets will lead to the formation of extreme helium stars, subdwarfs, or massive WDs. We identify three targets that are excellent gravitational wave sources. These should be detected by the Laser Interferometer Space Antenna like missions within the first year of operation. The remaining targets are important indicators of what the Galactic foreground may look like for gravitational wave observatories.

• Open/close Effect of Interacting Rarefaction Waves on Relativistically Hot Jets

  Jin Matsumoto et al. 2012 ApJ 751 140 Tag this article

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  The effect of rarefaction acceleration on the propagation dynamics and structure of relativistically hot jets is studied through relativistic hydrodynamic simulations. We emphasize the nonlinear interaction of rarefaction waves excited at the interface between a cylindrical jet and the surrounding medium. From simplified one-dimensional (1D) models with radial jet structure, we find that a decrease in the relativistic pressure due to the interacting rarefaction waves in the central zone of the jet transiently yields a more powerful boost of the bulk jet than that expected from single rarefaction acceleration. This leads to a cyclic in situ energy conversion between thermal and bulk kinetic energies, which induces radial oscillating motion of the jet. The oscillation timescale is characterized by the initial pressure ratio of the jet to the ambient medium and follows a simple scaling relation, τ _oscillation ( P _{jet, 0}/ P _{amb, 0}) ^1/2. Extended two-dimensional simulations confirm that this radial oscillating motion in the 1D system manifests as modulation of the structure of the jet in a more realistic situation where a relativistically hot jet propagates through an ambient medium. We find that when the ambient medium has a power-law pressure distribution, the size of the reconfinement region along the propagation direction of the jet in the modulation structure λ evolves according to a self-similar relation λ t ^α/2, where α is the power-law index of the pressure distribution.