Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.stsci.edu/~tbeck/research.pdf Дата изменения: Thu May 21 23:11:00 2009 Дата индексирования: Mon Oct 1 22:44:33 2012 Кодировка: Поисковые слова: ngc 4736

Tracy L. Beck
Research Interests and Plans
My res earc h interests are focus ed primarily on understanding t he formation and evolution of y oung stars, their circ umstellar disks and their ass ociat ed outflows of material. I have observ ed extens ively at optical through mid-infrared wavelengths at many ground-bas ed observ atories (wit h more than 130 nights obs erving at Gemini Nort h alone). I have experience reducing and analyzing data from ground and space based observatories (inc luding HST WFPC-2 and STIS) and hav e enjoyed us ing a wide range of astronomic al res ources for my studies - from very high spatial resolution infrared adaptive optics data obtained at 8-10 meter class observatories (Beck et al. 2004; Bec k et al. in prep.) t o 100 year-old photographic plates from the Harvard College Observatory archiv e that were acquired with a 1.5 inch refracting telescope (Beck & Simon, 2001). I am the primary inv estigat or for t wo projects that I plan to c ontinue in the fut ure but will mention only briefly here: 1) To gain a clearer understanding of how water-ice in disks around young stars is chemic ally and thermally proc essed by accretion activity, I hav e initiated a s urvey of the 2­4 µm s pectra of ~45 s tars obs erved through and embedded within the Taurus and Ophiuc hus dark cloud complex es (Beck 2006, submitted to AJ, Beck et al. in prep.) and 2) I have us ed very high spatial resolution imaging and spectrosc opy t o understand the multiplicity fraction and natures of binary stars in their early phases of formation (Beck, Simon & Close 2003; Beck et al. 2004b). Mass outflows of material are observ ed in sev eral environments in astronomy, from young stellar objects (YSOs ) to the distant nuclei of active galaxies. Studying outflows from the nearby YSOs provides the opportunity to examine jet structure at the highest obtainable spatial resolution. To date, it is unclear how mass inf all and accretion onto a prot ostar res ults in mass outflows seen as jets and Herbig-Haro (HH) objects. Howev er, the outflows likely regulat e accretion onto a star by removing exc ess angular momentum from the system. In this way, the efficiency of the outflows (or lack thereof) will likely determine the ultimate stellar mass. Thus, underst anding the inf all-outflo w proc ess and how it moderates mass accretion has become a fundamental goal of modern stellar astrophysics. In the past few years, I hav e become increasingly involv ed in investigations of the nature of YSO outflows using the comparably new t echnique of integral field spectrosc opy (Beck et al. 2004a; Beck et al. accepted to AJ, Beck et al. in prep). Integral field units (I FUs) provide spatially res olved imaging spectroscopy at optical and infrared wavelengths. By meas uring structure in spatially resolved emission lines, IFUs permit the direct study of kinematics, excitation states, and electron dens ities in YSO jets. For ex ample, line ratios of the optical Sulfur [S II] 6717 and 6731 Angstro m transitions provide a direct measure of the electron densities in a region. Spatially resolv ed I FU spectroscopy of the s pectac ular HH 34 jet have revealed a pronounc ed "striped" structure in electron density (Figure 1). This prov es for the first time that high electron density regions lie at the leading side of each outflow emission knot, whic h is in direct agreement with theoretical predictions of the structure of YSO outflows (Beck et al. accept ed for publication in AJ). Over t he c ours e of t he last decade, high spatial resolution HST and adaptiv e optics imaging studies hav e discovered well-collimated outflows and "mic ro jets" from YSOs. In fact, HST STIS spectral observations of a small subs et of these sources show evidence f or rotation in t he inner jet channels. The meas ured rat e of jet rotation is


roughly equivalent to the ex pected orbital velocit y of material in the inner circ umstellar disk, which implies there may be a connection bet ween the inner disk and outflow. Howev er, the marginal rotation signature found by stepping the STIS s pectral slit acros s the YSO jet has met with skepticis m, and alternative interpret ations also seem plausible. Confirmation and c haracteriz ation of jet rot ation signatures could provide important observational support of the theoretical "disk-wind" models that are adopted to explain the inner struct ure of YSO jets. The near infrared adaptive optics fed I FU, NI FS, at Gemini Nort h Observ atory provides imaging spectroscopy at resolutions as fine as ~14 AU toward nearby YSOs (0."1 at ~140pc dist ances ) and to <20k m/s velocit y accuracy. The specifications of NI FS are virtually identic al to STIS, and NI FS is presently the only instrument that can continue the difficult meas urements of jet rotation. As a result, I have initiated survey of YSO jets using NI FS in order to better understand the velocity structure in outflows and confirm rotation in the inner c hannels. Figure 2 pres ents the v elocity structure seen in forbidden [ Fe II] 1.644µm emission in the YSO jet associat ed with the young star HV Tau C. The velocity analysis shows a ~20k m/s shift across t he axis of the blue-shifted jet (designated by the red line), whic h could be indicative of rotation in the inner bow shock. Follow-up observ ations of this star at Gemini North are scheduled for Dec ember 2006 Director's Discretionary (DD) time to verify that this velocity shift is caused by rotation and not by short-liv ed turbulent motions in the inner jet channel. In the future, I plan t o continue using int egral field spectrosc opy (at Gemini and els ewhere) to study the velocity structure in the inner jet channels to confirm or refute the existing models and better understand how outflows moderate mass accretion onto prot ostars.