Five Colleges

Before the 2-Micron All-Sky Survey (2MASS) began, only six objects were known with spectral types later than M9.5 V. In the first 371 deg2 of actual 2MASS survey data, we have identified another 20 such objects spectroscopically confirmed using the Low Resolution Imaging Spectrograph (LRIS) at the W. M. Keck Observatory. Because the TiO and VO bands, which dominate the far-optical portions of late-M spectra, disappear in these cooler dwarfs, we define a new spectral class "L" in which metallic oxides are replaced by metallic hydrides and neutral alkali metals as the major spectroscopic signatures. We establish classification indices and type all 25 L dwarfs. The 26th "post-M9.5" object—Gl 229B—is the prototype of a methane-dominated spectral class, which we propose as class "T." At least five of the 20 2MASS L dwarfs show the 6708 Å lithium doublet at low resolution, the strongest having an equivalent width of 18.5 Å. For objects this cool, the presence of lithium proves that they are substellar. Two other 2MASS objects appear to have lithium lines at the limit of our detectability, which if verified means that at least one-third of our L dwarfs are bona fide brown dwarfs. All of the 2MASS brown dwarfs discovered so far have J-Ks > 1.30. We have not yet, despite deliberately searching for them, found any brown dwarfs with colors resembling Gl 229B (J-Ks ≈ -0.1).

We present near-infrared spectra for a sample of T dwarfs, including 11 new discoveries made using the 2 Micron All Sky Survey. These objects are distinguished from warmer (L-type) brown dwarfs by the presence of methane absorption bands in the 1-2.5 μm spectral region. A first attempt at a near-infrared classification scheme for T dwarfs is made, based on the strengths of CH_4 and H_2O bands and the shapes of the 1.25, 1.6, and 2.1 μm flux peaks. Subtypes T1 V-T8 V are defined, and spectral indices useful for classification are presented. The subclasses appear to follow a decreasing T_(eff) scale, based on the evolution of CH_4 and H_2O bands and the properties of L and T dwarfs with known distances. However, we speculate that this scale is not linear with spectral type for cool dwarfs, due to the settling of dust layers below the photosphere and subsequent rapid evolution of spectral morphology around T_(eff) ~ 1300-1500 K. Similarities in near-infrared colors and continuity of spectral features suggest that the gap between the latest L dwarfs and earliest T dwarfs has been nearly bridged. This argument is strengthened by the possible role of CH_4 as a minor absorber, shaping the K-band spectra of the latest L dwarfs. Finally, we discuss one peculiar T dwarf, 2MASS 0937+2931, which has very blue near-infrared colors (J - K_s = -0.89 ± 0.24) due to suppression of the 2.1 μm peak. The feature is likely caused by enhanced collision-induced H_2 absorption in a high-pressure or low-metallicity photosphere.

After studying how line width depends on spatial scale in low-mass star-forming regions, we propose that "dense cores" (Myers & Benson 1983) represent an inner scale of a self-similar process that characterizes larger scale molecular clouds.In the process of coming to this conclusion, we define four distinct types of line width-size relation (Delta upsilon proportional to R-ai), which have power-law slopes a(1), a(2), a(3), and a(4), as follows: Type 1-multitracer, multicloud intercomparison; Type 2-single-tracer, multicloud intercomparison; Type 3-multitracer study of a single cloud; and Type 4-single-tracer study of a single cloud. Type 1 studies (of which Larson 1981 is the seminal example) are compendia of Type 3 studies which illustrate the range of variation in the line width-size relation from one region to another.Using new measurements of the OH and (CO)-O-18 emission emanating from the environs of several of the dense cores studied in NH3 by Barranco & Goodman (1998; Paper I), we show that line width increases with size outside the cores with a(4) similar to 0.2. On scales larger than those traced by (CO)-O-18 or OH, (CO)-C-12 and (CO)-C-13 observations indicate that a(4) increases to similar to 0.5 (Heyer & Schloerb 1997). By contrast, within the half-power contour of the NH3 emission from the cores, line width is virtually constant, with a(4) similar to 0. We interpret the correlation between increasing density and decreasing Type 4 power-law slope as a "transition to coherence." Our data indicate that the radius R-coh at which the gas becomes coherent (i.e., a(4) --> 0) is of order 0.1 pc in regions forming primarily low-mass stars. The value of the nonthermal line width at which "coherence" is established is always less than but still of order of the thermal line width of H-2. Thus coherent cores are similar to, but not exactly the same as, isothermal balls of gas.Two other results bolster our proposal that a transition to coherence takes place at similar to 0.1 pc. First, the OH, (CO)-O-18, and NH3 maps show that the dependence of column density on size is much steeper (N proportional to R-0.9) inside R-coh than outside of it (N proportional to R-0.2), Which implies that the volume filling factor of coherent cores is much larger than in their surroundings. Second, Larson (1995) has recently found a break in the power law characterizing the clustering of stars in Taurus at 0.04 pc, just inside of R-coh. Larson and we interpret this break in slope as the point at which stellar clustering properties change from being determined by the (fractal) gas distribution (on scales greater than 0.04 pc) to being determined by fragmentation processes within coherent cores (on scales less than 0.04 pc).We speculate that the transition to coherence takes place when a dissipation threshold for the MHD turbulence that characterizes the larger scale medium is crossed at the critical inner scale R-coh. We suggest that the most likely explanation for this threshold is the marked decline in the coupling of the magnetic field to gas motions due to a decreased ion/neutral ratio in dense, high filling factor gas.

We have initiated a search for extended ultraviolet disk (XUV-disk) galaxies in the local universe. Here we compare GALEX UV and visible-NIR images of 189 nearby (D < 40 Mpc) S0-Sm galaxies included in the GALEX Atlas of Nearby Galaxies and present the first catalog of XUV-disk galaxies. We find that XUV-disk galaxies are surprisingly common but have varied relative (UV/optical) extent and morphology. Type 1 objects (≳20% incidence) have structured, UV-bright/optically faint emission features in the outer disk, beyond the traditional star formation threshold. Type 2 XUV-disk galaxies (~10% incidence) exhibit an exceptionally large, UV-bright/optically low surface brightness (LSB) zone having blue UV-K_s outside the effective extent of the inner, older stellar population, but not reaching extreme galactocentric distance. If the activity occurring in XUV-disks is episodic, a higher fraction of present-day spirals could be influenced by such outer disk star formation. Type 1 disks are associated with spirals of all types, whereas Type 2 XUV-disks are predominantly found in late-type spirals. Type 2 XUV-disks are forming stars quickly enough to double their (currently low) stellar mass in the next Gyr (assuming a constant star formation rate). XUV-disk galaxies of both types are systematically more gas-rich than the general galaxy population. Minor external perturbation may stimulate XUV-disk incidence, at least for Type 1 objects. XUV-disks are the most actively evolving galaxies growing via inside-out disk formation in the current epoch, and may constitute a segment of the galaxy population experiencing significant, continued gas accretion from the intergalactic medium or neighboring objects.

The anisotropy of the cosmic microwave background radiation contains information about the contents and history of the universe. We report new limits on cosmological parameters derived from the angular power spectrum measured in the first Antarctic flight of the Boomerang experiment. Within the framework of models with adiabatic perturbations, and using only weakly restrictive prior probabilities on the age of the universe and the Hubble expansion parameter h, we find that the curvature is consistent with flat and that the primordial fluctuation spectrum is consistent with scale invariant, in agreement with the basic inflation paradigm. We find that the data prefer a baryon density ${\ensuremath{\Omega}}_{b}{h}^{2}$ above, though similar to, the estimates from light element abundances and big bang nucleosynthesis. When combined with large scale structure observations, the Boomerang data provide clear detections of both dark matter and dark energy contributions to the total energy density ${\ensuremath{\Omega}}_{\mathrm{tot}},$ independent of data from high-redshift supernovae.

We report on observations of Pab and Brc for a sample of classical T Tauri stars in Taurus and nd a tight correlation between the emission-line luminosities and the accretion luminosity as measured from the hot continuum excess. We use the Brc luminosity correlation to calculate accretion luminosities in highly reddened young stars with existing line measurements. The distribution of accretion luminosities is similar in Taurus and Ophiuchus Class II sources. For the deeply embedded Class I objects, the accretion luminosities are in general less than the bolometric luminosities, which implies that the disk accretion rates are signicantly lower than the envelope infall rates. We nd that the central sources of many Class I objects are quite similar to their Class II counterparts.

A summary of global properties and an evaluation of the equilibrium state of molecular regions in the outer Galaxy are presented from the decomposition of the FCRAO Outer Galaxy Survey and targeted 12CO and 13CO observations of four giant molecular cloud complexes. The ensemble of identified objects includes both small, isolated clouds and clumps within larger cloud complexes. The 12CO luminosity function and size distribution of a subsample of objects with well-defined distances are determined such that ΔN/ΔLCO = L and ΔN/Δre = r. 12CO velocity dispersions show little variation with cloud sizes for radii less than 10 pc. It is demonstrated that the internal motions of regions with MCO = XCOLCO > 104 M are bound by self-gravity, yet the constituent clumps of cloud complexes and isolated molecular clouds with MCO < 103 M are not in self-gravitational equilibrium. The required external pressures to maintain the equilibrium of this population are (1-2) × 104 cm-3 K.

Using a statistical equilibrium radiative transfer treatment, we examine the behavior of hydrogen emission lines arising from the magnetospheric infall zones of classical T Tauri stars. Having calculated self-consistent line profiles of the Balmer lines, Paβ and Brγ, we explore parameter space, examining the effects of the magnetospheric gas temperature and size on the line fluxes. We compare model and observed line fluxes for the Balmer lines and find a good match using a relatively small range of parameters. We are also able to match the observed Brγ line profile of the embedded object WL 16, which supports the use of the infrared lines in studying magnetospheric infall even in the earliest stages of star formation. Finally, we discuss constraints on the physical parameters and the possibility of using the emission lines as accretion rate indicators.

241-+

We present a comprehensive study of the physical and chemical conditions along the TMC-1 ridge. Temperatures were estimated from observations of CH3CCH, NH3, and CO. Densities were obtained from a multitransition study of HC3N. The values of the density and temperature allow column densities for 13 molecular species to be estimated from statistical equilibrium calculations, using observations of rarer isotopomers where possible, to minimize opacity effects. The most striking abundance variations relative to HCO+ along the ridge were seen for HC3N, CH3CCH, and SO, while smaller variations were seen in CS, C2H, and HCN. On the other hand, the NH3, HNC, and N2H+ abundances relative to HCO+ were determined to be constant, indicating that the so-called NH3 peak in TMC-1 is probably a peak in the ammonia column density rather than a relative abundance peak. In contrast, the well-studied cyanopolyyne peak is most likely due to an enhancement in the abundance of long-chain carbon species. Comparisons of the derived abundances to the results of time-dependent chemical models show good overall agreement for chemical timescales around 10(5) yr. We find that the observed abundance gradients can be explained either by a small variation in the chemical timescale from 1.2 x 10(5) to 1.8 x 10(5) yr or by a factor of 2 change in the density along the ridge. Alternatively, a variation in the C/O ratio from 0.4 to 0.5 along the ridge produces an abundance gradient similar to that observed.

We present high-resolution spectra of classical T Tauri stars in Taurus in the range 5800–9000 Å to critically examine the current theories of magnetospheric accretion. In this paper, we focus on the emission lines of hydrogen (the higher Paschen series and Hα), He I λ5876, O I λ7773 and λ8446, the Na D lines, and the Ca II infrared triplet. We argue that the magnetospheric infall zone is the likely source of emission for the hydrogen, oxygen, and sodium lines for most of the stars in our sample. As has been previously shown, the calcium and helium lines have a narrow and a broad component; focusing on the broad components, we find that they are likely formed in the magnetosphere for at least several of the stars in the sample. Preliminary calculations indicate that the magnetospheric models developed to explain the Balmer lines predict significant Ca II, Paschen, and O I emission, roughly comparable to what is observed. However, in some of the stars, especially at high veilings, the case for line formation in the accretion flow is not clear. Finally, our results suggest that the Ca II IR triplet lines are good indicators of the accretion rate for all but the most weakly accreting T Tauri stars.

We report the discovery of a widely separated (258&farcs;3+/-0&farcs;4) T dwarf companion to the Gl 570ABC system. This new component, Gl 570D, was initially identified from the Two Micron All-Sky Survey. Its near-infrared spectrum shows the 1.6 and 2.2 µm CH4 absorption bands characteristic of T dwarfs, while its common proper motion with the Gl 570ABC system confirms companionship. Gl 570D (MJ=16.47+/-0.07) is nearly a full magnitude dimmer than the only other known T dwarf companion, Gl 229B, and estimates of L=&parl0;2.8+/-0.3&parr0;x10-6 L middle dot in circle and Teff=750+/-50 K make it significantly cooler and less luminous than any other known brown dwarf companion. Using evolutionary models by Burrows et al. and an adopted age of 2-10 Gyr, we derive a mass estimate of 50+/-20 MJup for this object.

In a high dispersion 1 micron survey of 39 classical T Tauri stars veiling is detected in 80%, and He I 10830 and Pgamma line emission in 97% of the stars. On average, the 1 micron veiling exceeds the level expected from previously identified sources of excess emission, suggesting the presence of an additional contributor to accretion luminosity in the star-disk interface region. Strengths of both lines correlate with veiling, and at Pgamma there is a systematic progression in profile morphology with veiling. He I 10830 has an unprecedented sensitivity to inner winds, showing blueshifted absorption below the continuum in 71% of the CTTS compared to 0% at pgamma. This line is also sensitive to magnetospheric accretion flows, with redshifted absorption below the continuum found in 47% of the CTTS compared to 24% at Pgamma. The blueshifted absorption at 10830 shows considerable diversity in its breadth and penetration depth into the continuum, indicating that a range of inner wind conditions exist in accreting stars. We interpret the broadest and deepest blue absorptions as formed from scattering of the 1 micron continuum by outflowing gas whose full acceleration region envelopes the star, suggesting radial outflow from the star. In contrast, narrow blue absorption with a range of radial velocities more likely arises via scattering of the 1 micron continuum by a wind emerging from the inner disk. Both stellar and disk winds are accretion powered since neither is seen in non-accreting WTTS and among the CTTS helium strength correlates with veiling.

We present extensive optical and infrared photometry of the afterglow of gamma-ray burst (GRB) 030329 and its associated supernova (SN) 2003dh over the first two months after detection (2003 March 30-May 29 UT). Optical spectroscopy from a variety of telescopes is shown and, when combined with the photometry, allows an unambiguous separation between the afterglow and supernova contributions. The optical afterglow of the GRB is initially a power-law continuum but shows significant color variations during the first week which are unrelated to the presence of a supernova. The early afterglow light curve also shows deviations from the typical power-law decay. A supernova spectrum is first detectable ~7 days after the burst and dominates the light after ~11 days. The spectral evolution and the light curve are shown to closely resemble those of SN 1998bw, a peculiar Type Ic SN associated with GRB 980425, and the time of the supernova explosion is very close to the observed time of the GRB. It is now clear that at least some GRBs arise from core-collapse SNe.

We present a study of the structure of four molecular clouds from the Milky Way Galactic Ring Survey (GRS), a Boston University and Five College Radio Astronomy Observatory collaboration. The GRS is a new high-resolution survey in the 13CO J = 1 → 0 spectral line of the inner Galaxy and the 5 kpc ring, the Milky Way's dominant star-forming structure. Because of the smaller line widths of 13CO compared to 12CO, we can avoid velocity crowding and establish accurate kinematic distances to the clouds. The kinematic distance ambiguity in the first Galactic quadrant is resolved using self-absorption in complementary high-resolution atomic hydrogen data. The four clouds are selected to span a large range of star formation activity, from the quiescent cloud GRSMC 45.60+0.30, which shows no signs of high-mass star formation, to W49, the most luminous star-forming region in the Galaxy. We use a three-dimensional Gaussian clump decomposition to identify clumps in the clouds and to investigate their properties. Each cloud has the same clump mass spectrum, dN/dM ∝ M-1.8, independent of star formation activity. We do not find significant differences in the slopes of the relations of density, line width, and clump mass as a function of clump size among the clouds. The size-density and size-line width relations show considerable scatter. Compared to the conventional Larson scaling laws, we find systematically flatter slopes for the size-density and size-line width relations and a higher power-law index for the size-mass relation. In particular, the clump line widths for the most quiescent cloud GRSMC 45.60+0.30 are independent of clump size. While the clouds as a whole are gravitationally bound, most of the clumps are not; only a small fraction of the total number of clumps is self-gravitating. The active star-forming clouds have a higher fraction of gravitationally bound clumps and a higher mean cloud volume density than the more quiescent clouds. The gravitationally unbound clumps are possibly confined by the weight of the self-gravitating complex. The pressures needed to bind these clumps are largest for the active star-forming clouds, which have a much higher weight than the quiescent clouds. Alternatively, a high number of the gravitationally unbound clumps may be transient.

We have found that two members of the TW Hydrae association, TW Hydrae and Hen 3-600A, are still actively accreting, based on the ballistic infall signature of their broad Halpha emission profiles. We present the first quantitative analysis of accretion in these objects and conclude that the same accretion mechanisms which operate in the well-studied 1 Myr old T Tauri stars can and do occur in older (10 Myr) stars. We derive the first estimates of the disk mass accretion rate in TW Hya and Hen 3-600A, which are 1-2 orders of magnitude lower than the average rates in 1 Myr old objects. The decrease in accretion rates over 10 Myr, as well as the low fraction of TW Hya association objects still accreting, points to significant disk evolution, possibly linked to planet formation. Given the multiplicity of the Hen 3-600 system and the large UV excess of TW Hya, our results show that accretion disks can be surprisingly long lived in spite of the presence of companions and significant UV ionizing flux.

Journal Article A "Post-Gay" Era? Media Gaystreaming, Homonormativity, and the Politics of LGBT Integration Get access Eve Ng Eve Ng 1Five College Women's Studies Research Center, South Hadley, MA 01075, USA2Department of Communication, Machmer Hall, University of Massachusetts, Amherst, MA, 01003-9278 Search for other works by this author on: Oxford Academic Google Scholar Communication, Culture and Critique, Volume 6, Issue 2, June 2013, Pages 258–283, https://doi.org/10.1111/cccr.12013 Published: 10 May 2013

We report the discovery of four field methane ("T"-type) brown dwarfs using Two Micron All-Sky Survey (2MASS) data. One additional methane dwarf, previously discovered by the Sloan Digital Sky Survey, was also identified. Near-infrared spectra clearly show the 1.6 and 2.2 m CH4 absorption bands characteristic of objects with Teff 1300 K as well as broadened H2O bands at 1.4 and 1.9 m. Comparing the spectra of these objects with that of Gl 229B, we propose that all new 2MASS T dwarfs are warmer than 950 K, in order from warmest to coolest: 2MASS J1217-03, 2MASS J1225-27, 2MASS J1047+21, and 2MASS J1237+65. Based on this preliminary sample, we find a warm T dwarf surface density of 0.0022 T dwarfs deg-2, or 90 warm T dwarfs over the whole sky detectable to J < 16. The resulting space density upper limit, 0.01 T dwarfs pc-3, is comparable to that of the first L dwarf sample from Kirkpatrick et al.

We compare accretion shock models with optical and ultraviolet spectra of pre-main sequence stars to (1) make the first determinations of accretion rates in intermediate mass T Tauri stars from continuum emission, and (2) derive improved estimates of accretion rates and extinctions for continuum T Tauri stars. Our method extends the shock models developed by Calvet and Gullbring to enable comparisons with optical and archival International Ultraviolet Explorer ultraviolet spectra. We find good agreement between the observations and the model predictions, supporting the basic model of magnetospheric accretion shocks as well as previous determinations of accretion rates and interstellar reddening for the low mass T Tauri stars. The accretion rates determined for the intermediate-mass T Tauri stars agree well with values obtained through other methods that use near-infrared hydrogen line strengths (Muzerolle et al. 1998).

We present and analyse spectra of the Type IIn supernova (SN) 1994W obtained between 18 and 203 d after explosion. During the luminous phase (first 100 d) the line profiles are composed of three major components: (i) narrow P-Cygni lines with the absorption minima at -700 km s -1 ; (ii) broad emission lines with blue velocity at zero intensity 4000 km s -1 ; and (iii) broad, smooth wings extending out to at least 5000 km s -1 , most apparent in H. These components are identified with an expanding circumstellar (CS) envelope, shocked cool gas in the forward post-shock region, and multiple Thomson scattering in the CS envelope, respectively. The absence of broad P-Cygni lines from the SN is the result of the formation of an optically thick, cool, dense shell at the interface of the ejecta and the CS envelope. Models of the SN deceleration and Thomson scattering wings are used to recover the density (n 10 9 cm -3 ), radial extent [(4-5) 10 15 cm] and Thomson optical depth ( T 2.5) of the CS envelope during the first month. The plateau-like SN light curve is reproduced by a hydrodynamical model and is found to be powered by a combination of internal energy leakage after the explosion of an extended pre-SN (10 15 cm) and subsequent luminosity from CS interaction. The pre-explosion kinematics of the CS envelope is recovered, and is close to homologous expansion with outer velocity 1100 km s -1 and a kinematic age of 1.5 yr. The high mass (0.4 M ) and kinetic energy (2 10 48 erg) of the CS envelope, combined with low age, strongly suggest that the CS envelope was explosively ejected 1.5 yr prior to the SN explosion.