César A. Briceño

Disk Accretion Rates for T Tauri Stars

We present new measurements of disk accretion rates for T Tauri stars in the Taurus molecular cloud complex. Our results are based on intermediate-resolution spectrophotometry from 3200 to 5200 A, which is used to derive the excess hot continuum emission produced by accretion onto the central star. Previous estimates of T Tauri accretion rates in the literature differ by as much as 1 order of magnitude; our measurements agree better with the lowest estimates, and we discuss the problems and systematic effects that led to the previous disagreement. In particular, we note that the stellar photospheric emission from nonaccreting T Tauri stars exhibits color anomalies compared to main-sequence stars; these anomalies make the estimated extinction depend upon the color index used. We argue that the V-R index is a reasonable compromise to match with optically derived spectral types, and that V-I and V-J are much more likely to be biased by cooler companion stars and starspots. We develop a calibration with which approximate mass accretion rates can be derived for T Tauri stars based on broadband photometry and spectral types, which should enable accretion rates to be estimated for large samples with greater ease.

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.

Ubvri light curves of 44 type ia supernovae

We present UBVRI photometry of 44 Type Ia supernovae (SNe Ia) observed from 1997 to 2001 as part of a continuing monitoring campaign at the Fred Lawrence Whipple Observatory of the Harvard-Smithsonian Center for Astrophysics. The data set comprises 2190 observations and is the largest homogeneously observed and reduced sample of SNe Ia to date, nearly doubling the number of well-observed, nearby SNe Ia with published multicolor CCD light curves. The large sample of U-band photometry is a unique addition, with important connections to SNe Ia observed at high redshift. The decline rate of SN Ia U-band light curves correlates well with the decline rate in other bands, as does the U - B color at maximum light. However, the U-band peak magnitudes show an increased dispersion relative to other bands even after accounting for extinction and decline rate, amounting to an additional ~40% intrinsic scatter compared to the B band.

Spectral analysis and classification of herbig Ae/Be stars

We present an analysis of the optical spectra of 75 early-type emission-line stars, many of which have been classified previously as Herbig Ae/Be (HAeBe) stars. Accurate spectral types were derived for 58 members of the sample; high continuum veiling, contamination by nonphotospheric absorption features, or a composite binary spectrum prevented accurate spectral typing for the rest. Approximately half of our sample exhibited [O I] λ6300 forbidden-line emission down to our detection limit of 0.1 A equivalent width; a third of the sample exhibited Fe II emission (multiplet 42). A subset of 11 of the HAeBe sample showed abnormally strong Fe II absorption; 75% of this subset are confirmed UX Ori objects. Combining our spectral typing results with photometry from the literature, we confirm previous findings of high values of total-to-selective extinction (RV ~ 5) in our larger sample, suggesting significant grain growth in the environments of HAeBe stars. With this high value of RV, the vast majority of HAeBe stars appear younger than with the standard RV = 3.1 extinction law and are more consistent with being pre–main-sequence objects.

A Search for Very Low Mass Pre-Main-Sequence Stars in Taurus*

?????We present the initial results of a deep CCD survey for very low mass pre?main-sequence (PMS) stars in selected fields of the Taurus molecular cloud complex. The results reported herein span a little over half a square degree in the dark clouds L1495, L1529, L1551, and B209. Our survey is complete down to IC ~ 19 at (R - I)C ~ 2.5, which enables us to probe well below the hydrogen burning limit at 1?2 Myr. From follow-up spectroscopic observations we have identified nine new low-mass T Tauri stars (TTSs). A large fraction of the new PMS objects (5/9) have very low masses as inferred from their late spectral types (?M5), and comparison with recent evolutionary tracks and Pleiades brown dwarfs suggests that our M6?M6.5 new TTSs are very young brown dwarfs. Two of the new TTSs may constitute a new, moderately embedded, binary classical T Tauri system. The new young stars represent a ~38% increase in the known PMS population of our survey area and a factor of ~2 increase in the number of known late-type TTSs. In spite of our sensitivity, we detect no young stars with spectral types later than ~M7. Our results illustrate the importance of spectroscopy in eliminating foreground M stars.

Inducible Clustering of Membrane-Targeted SH3 Domains of the Adaptor Protein Nck Triggers Localized Actin Polymerization

BACKGROUND SH2/SH3 adaptor proteins play a critical role in tyrosine kinase signaling pathways, regulating essential cell functions by increasing the local concentration or altering the subcellular localization of downstream effectors. The SH2 domain of the Nck adaptor can bind tyrosine-phosphorylated proteins, while its SH3 domains can modulate actin polymerization by interacting with effectors such as WASp/Scar family proteins. Although several studies have implicated Nck in regulating actin polymerization, its role in living cells is not well understood. RESULTS We used an antibody-based system to experimentally modulate the local concentration of Nck SH3 domains on the plasma membrane of living cells. Clustering of fusion proteins containing all three Nck SH3 domains induced localized polymerization of actin, including the formation of actin tails and spots, accompanied by general cytoskeletal rearrangements. All three Nck SH3 domains were required, as clustering of individual SH3 domains or a combination of the two N-terminal Nck SH3 domains failed to promote significant local polymerization of actin in vivo. Changes in actin dynamics induced by Nck SH3 domain clustering required the recruitment of N-WASp, but not WAVE1, and were unaffected by downregulation of Cdc42. CONCLUSIONS We show that high local concentrations of Nck SH3 domains are sufficient to stimulate localized, Cdc42-independent actin polymerization in living cells. This study provides strong evidence of a pivotal role for Nck in directly coupling ligand-induced tyrosine phosphorylation at the plasma membrane to localized changes in organization of the actin cytoskeleton through a signaling pathway that requires N-WASp.

A Young Star near the Hydrogen-burning Limit

We show that V410 X-ray 3, toward the L1495E star-forming complex, is an object with a mass of 0.08-0.15 M☉ and an age of ~1 Myr. Nonetheless, it has emerged from its natal cloud and can be studied in detail throughout the optical and near-infrared, providing new insights into the character of very young and low-mass objects. It has spectral characteristics intermediate between those of late dwarfs and giants (e.g., first-overtone CO typical of an M6 dwarf, but K I, Na I, and TiO/VO typical of an M6 giant and CaH intermediate between the luminosity classes). Its optical and IR photometric colors are consistent with those of an M6 dwarf. If the latest theoretical evolutionary tracks are valid at young ages and low masses, it appears that the hydrogen-burning limit at an age of ~1 Myr occurs at a spectral type of M6-M7.

THE GOULD’S BELT DISTANCES SURVEY (GOBELINS). II. DISTANCES AND STRUCTURE TOWARD THE ORION MOLECULAR CLOUDS

We present the results of the Goulds Belt Distances Survey of young star-forming regions toward the Orion Molecular Cloud Complex. We detected 36 young stellar objects (YSOs) with the Very Large Baseline Array, 27 of which have been observed in at least three epochs over the course of two years. At least half of these YSOs belong to multiple systems. We obtained parallax and proper motions toward these stars to study the structure and kinematics of the Complex. We measured a distance of 388 ± 5 pc toward the Orion Nebula Cluster, 428 ± 10 pc toward the southern portion L1641, 388 ± 10 pc toward NGC 2068, and roughly ~420 pc toward NGC 2024. Finally, we observed a strong degree of plasma radio scattering toward λ Ori.

First results of the CIDA Schmidt survey: selected zones in Taurus-Auriga

We have begun an objective-prism H-alpha survey of star-forming regions using the CIDA 1 m Schmidt Camera, with a limiting magnitude of V ~ 18. We report here first results for selected areas of the Taurus-Auriga molecular clouds. Of the list of candidates found in the objective-prism plates, 12 stars have been confirmed as pre-main sequence by the detection of the Li I 6707 A absorption line. Five of these stars are in the dark cloud L1544, where only one T Tauri star was previously known. The new stars have very late spectral types and most have estimated masses between 0.2 M and 0.3 M.

A Large-Scale Objective-Prism and X-Ray Survey in Taurus-Auriga* **

We present the results of a moderately deep, wide-field optical/X-ray survey extending over ~60° in the Taurus-Auriga molecular cloud complex. Our observations are sensitive enough to allow us to detect lower mass and older pre–main-sequence stars that could have been missed in previous surveys. We identify four new T Tauri stars, two in the area of L1500 and L1503, one in L1538, and one in L1544. X-rays were detected from three near-IR sources, one of them a very reddened Class II T Tauri star. Four X-ray sources have no known optical/IR known counterpart; they could be very reddened pre–main-sequence stars. Our findings are consistent with the idea that little (if any) star formation has taken place in L1537 and L1538. Although we found one slightly older T Tauri star (~3–4 Myr old), our survey yields no evidence for any significant number of T Tauri stars, regardless of mass and age, outside the main groups of young stars in Taurus. We conclude that the M stars comprising the low-mass end of the ROSAT All-Sky Survey (RASS) source population must be older than ~10 Myr and most probably unrelated to the ongoing process of star formation in Taurus. Instead, the bulk of the RASS sources seen toward regions like Taurus seem to trace the history of star formation over a longer period in the solar vicinity.