Priscilla J. Benson

Dense cores in dark clouds. VIII: Velocity gradients

An analysis of motions consistent with uniform rotation in dense cores is presented. Twenty-nine of the 43 cores studied have a statistically significant gradient. Some gradients are spatially continuous and are consistent with uniform rotation, but other apparent gradients are caused by clump-clump motion, or sharp localized gradients, within a map. The motions in L1495, B217, L1251, L43, B361, and L1551 are discussed in detail. In L1551, the residuals of the fit to the NH3 velocity field indicate an outflow from IRS5 in the same direction as the CO outflow. Gradient orientation appears to be preserved over a range of density, as evidenced by comparing results of NH3 to fits of (C-18)O and CS maps. The specific angular momentum is found to scale roughly as F exp 3/2, where R represents the diameter of the FWHM intensity contour in a map.

BVRI Light Curves for 22 Type 1a Supernovae

We present 1210 Johnson/Cousins B, V, R, and I photometric observations of 22 recent Type Ia supernovae (SNe Ia): SNe 1993ac, 1993ae, 1994M, 1994S, 1994T, 1994Q, 1994ae, 1995D, 1995E, 1995al, 1995ac, 1995ak, 1995bd, 1996C, 1996X, 1996Z, 1996ab, 1996ai, 1996bk, 1996bl, 1996bo, and 1996bv. Most of the photometry was obtained at the Fred Lawrence Whipple Observatory of the Harvard-Smithsonian Center for Astrophysics in a cooperative observing plan aimed at improving the database for SNe Ia. The redshifts of the sample range from cz = 1200 to 37,000 km s-1 with a mean of cz = 7000 km s-1.

L1544: A Starless Dense Core with Extended Inward Motions

We present a multiline study of the dense core L1544 in the Taurus molecular complex. Although L1544 does not harbor an embedded star, it presents several characteristics of cores that have already undergone star formation, suggesting that it may be rather advanced in its evolution toward becoming a star-forming core. The spectral lines from L1544 present an interesting dichotomy, with the thick dense gas tracers su†ering very strong self absorption while CO and its isotopes are not being absorbed at all. The presence of the self absorptions allows us to study both the density structure and kinematics of the gas in detail. A simple analysis shows that the core is almost isothermal and that the self absorptions are due to very subthermal excitation of the dense gas tracers in the outer layers. The density has to decrease outward rapidly, and a detailed radiative transfer calculation that simultaneously -ts three iso- topes of CO and two of CS shows that the density approximately follows a r~1.5 power law. The self absorptions, in addition, allow us to measure the relative velocity between the inner and outer layers of the core, and we -nd that there is a global pattern of inward motions (background and foreground approaching each other). The relative speed between the foreground and background changes with posi- tion, and we use a simple two-layer model to deduce that while the foreground gas has a constant veloc- ity, the background material presents systematic velocity changes that we interpret as arising from two velocity components. We explore the origin of the inward motions by comparing our observations with models of gravitational collapse. A model in which the infall starts at the center and propagates outward (as in the inside-out collapse of Shu) is inconsistent with the large extension of the absorption (that sug- gests an advanced age) and the lack of a star at the core center (that suggests extreme youth). Ambipolar di†usion seems also ruled out because of the large amount of the inward speed (up to 0.1 km s~1) and the fact that ionized species move with speeds similar to those of the neutrals. Other infall models seem also to have problems -tting the data, so if L1544 is infalling, it seems to be doing so in a manner not contemplated by the standard theories of star formation. Our study of L1544 illustrates how little is still known about the physical conditions that precede star formation and how detailed studies of starless cores are urgently needed. Subject headings: ISM: individual (L1544) E ISM: kinematics and dynamics E stars: formation

Near-infrared and optical observations of IRAS sources in and near dense cores

This paper reports observations of 34 IRAS sources associated with dense cores in dark clouds at wavelengths 0.4-20 microns. The stars near cores tend to be visible T Tauri stars, while stars in cores tend to have circumstellar extinction 30-90 mag and luminosity about one solar, similar to that of T Tauri stars. The typical highly obscured star is probably accompanied by a luminous structure of substellar temperature, such as a circumstellar disk. In Taurus-Auriga, stars in cores probably become visible T Tauri stars less than 100,000 yr after they become detectable by IRAS; i.e., after they attain luminosity greater than about 0.1 solar. This implies that they are extremely young and may still be accreting. 54 references.

The Type Ia Supernova 1998bu in M96 and the Hubble Constant

We present optical and near-infrared photometry and spectroscopy of the Type Ia SN 1998bu in the Leo I Group galaxy M96 (NGC 3368). The data set consists of 356 photometric measurements and 29 spectra of SN 1998bu between UT 1998 May 11 and July 15. The well-sampled light curve indicates the supernova reached maximum light in B on UT 1998 May 19.3 (JD 2450952.8 ± 0.8) with B = 12.22 ± 0.03 and V = 11.88 ± 0.02. Application of a revised version of the Multicolor Light Curve Shape (MLCS) method yields an extinction toward the supernova of AV = 0.94 ± 0.15 mag, and indicates the supernova was of average luminosity compared to other normal Type Ia supernovae. Using the HST Cepheid distance modulus to M96 and the MLCS fitted parameters for the supernova, we derive an extinction-corrected absolute magnitude for SN 1998bu at maximum, MV = -19.42 ± 0.22. Our independent results for this supernova are consistent with those of Suntzeff et al. Combining SN 1998bu with three other well-observed local calibrators and 42 supernovae in the Hubble flow yields a Hubble constant, H0 = 64 -->img1.gif km s-1 Mpc-1, where the error estimate incorporates possible sources of systematic uncertainty including the calibration of the Cepheid period-luminosity relation, the metallicity dependence of the Cepheid distance scale, and the distance to the LMC.

Dense Cores in Dark Clouds. XI. A Survey for N2H+, C3H2, and CCS

We have used the 3 mm receiver and upgraded antenna of Haystack Observatory to make high spatial and spectral resolution observations of lines of N2H+ and C3H2 in 60 dense cores. Both species are detected in most of the cores, and the velocities, and line widths are well correlated. This suggests that ions and neutrals are well coupled. We found vD,max ? 0.03 km s-1, which is the first observational constraint on the relative speed of ions and neutrals in star-forming dense cores. Twenty of the cores were also observed in the JN = 43-32 line of CCS. From our N2H+ observations, the fractional abundance of the molecular nitrogen N2 is found to be 7 ? 10-5, consistent with all the nitrogen being in molecular form and not depleted onto dust grains.

CCD photometry of three Type Ia supernovae: V, R, and I light curves

Cousins system VRI photometry has been obtained for the Type Ia supernova 1992 G with CCDs attached to small telescopes at Leuschner Observatory (LO), Van Vleck Observatory (VVO), and Whitin Observatory (WO). Similar equipment, observing and reduction procedures have been adopted at all three sites and there is generally good agreement among the data sets. Two other Type Ia supernovae, 1991 M and 1991 T, were observed only at VVO. The shapes of the light curves of all three supernovae, including the spectroscopically peculiar SN 1991 T, are quite similar, and the combined light curve in V matches Leibundguts [Ph.D. thesis, University of Basel (1988)] template reasonably well

Observations of HC/sub 5/N and NH/sub 3/ in Taurus

Observations of HC/sub 5/N lines toward TMC-2 indicate that it is a small (Lapprox.0.1 pc), dense (napprox.4 x 10/sup 4/ cm/sup -3/), low-mass (Mapprox.1 M/sub sun/) fragment in the Taurus complex, with velocity dispersion at the emission peak only about twice thermal (..delta..vapprox.0.2 km s/sup -1/). The HC/sub 5/N emission region in TMC-2 has roughly half the projected area of that in TMC-1, and is more round than filamentary. The HC/sub 5/N and NH/sub 3/ emission regions in TMC-2 are coincident, with N (HC/sub 5/N)/N (NH/sub 3/) approx.0.1. The line width is much smaller than the free-fall width; the deduced values of L, n, and T satisfy the virial-theorem requirement for stable equilibrium. The temporary equilibrium of such fragments may serve to lengthen the time scales for formation of low-mass stars and long-chain molecules.

Water masers associated with two carbon stars: EU Andromedae and V778 Cygni

We observed the 22.2 GHz H2O maser line from the two C stars EU And and V778 Cyg. The intensity of the line has varied by more than a factor of 5 over several month and the line has shown additional weaker components. We interpret the system as being binary with a C and an M star component with a thick shell.

The OTHER Keck Observatories

The W. M. Keck Foundation is well known in astronomical circles for funding major telescopes in Hawaii. Less well known is the Foundation’s support of astronomy in a group of eight liberal arts colleges, the Keck Northeast Astronomy Consortium (KNAC). Funds from the Keck Foundation enabled the eight colleges to purchase CCD cameras and to upgrade telescope and computer facilities. More importantly, KNAC devised an exchange program for summer research students that allows students to follow up their astronomical interests at any of the other colleges except their home campus, thus providing them with a choice of up to two dozen faculty members to work with, rather than one or two. “KNAC students” report on their summer research at an annual symposium. A substantial number of student co-authored journal papers have emerged from the KNAC program. KNAC students have been very successful in graduate school admissions and in their later scientific careers.