Browsing by Subject "compact"

Now showing 1 - 4 of 4

Discovery of A ZZ Ceti in the Kepler Mission Field
(2011-11) Hermes, J. J.; Mullally, Fergal; Ostensen, R. H.; Williams, Kurtis A.; Telting, John; Southworth, John; Bloemen, S.; Howell, Steve B.; Everett, Mark; Winget, D. E.; Hermes, J. J.; Winget, D. E.
We report the discovery of the first identified pulsating DA white dwarf, WD J1916+3938 (Kepler ID 4552982), in the field of the Kepler mission. This ZZ Ceti star was first identified through ground-based, time-series photometry, and follow-up spectroscopy confirms that it is a hydrogen-atmosphere white dwarf with T(eff) = 11,129 +/- 115 K and log g = 8.34 +/- 0.06, placing it within the empirical ZZ Ceti instability strip. The object shows up to 0.5% amplitude variability at several periods between 800 and 1450 s. Extended Kepler observations of WD J1916+3938 could yield the best light curve, to date, of any pulsating white dwarf, allowing us to directly study the interior of an evolved object representative of the fate of the majority of stars in our Galaxy.
HST-COS Observations Of AGNs. II. Extended Survey Of Ultraviolet Composite Spectra From 159 Active Galactic Nuclei
(2014-10) Stevans, Matthew L.; Shull, J. Michael; Danforth, Charles W.; Tilton, Evan M.; Stevans, Matthew L.
The ionizing fluxes from quasars and other active galactic nuclei (AGNs) are critical for interpreting their emissionline spectra and for photoionizing and heating the intergalactic medium. Using far-ultraviolet (FUV) spectra from the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST), we directly measure the rest-frame ionizing continua and emission lines for 159 AGNs at redshifts 0.001 < Z(AGN) < 1.476 and construct a composite spectrum from 475 to 1875 A. We identify the underlying AGN continuum and strong extreme ultraviolet (EUV) emission lines from ions of oxygen, neon, and nitrogen after masking out absorption lines from the HI Ly alpha forest, 7 Lyman-limit systems (N-HI, 10(17.2) Cm-2) and 214 partial Lyman-limit systems (14.5 < log N-HI < 17.2). The 159 AGNs exhibit a wide range of FUV/EUV spectral shapes, F-V, proportional to v(proportional to v)(4, typically with -2 <= alpha(v), <= 0 and no discernible continuum edges at 912 angstrom (HI) or 504 angstrom (He I). The composite rest-frame continuum shows a gradual break at lambda(br) approximate to 1000 angstrom, with mean spectral index alpha(v) = -0.83 +/- 0.09 in the FUV (1200-2000 angstrom) steepening to alpha(v), = -1.41 +/- 0.15 in the EUV (500-1000 angstrom). We discuss the implications of the UV flux turnovers and lack of continuum edges for the structure of accretion disks, AGN mass inflow rates, and luminosities relative to Eddington values.
Ionized Gas Kinematics At High Resolution. II. Discovery Of A Double Infrared Cluster In II Zw 40
(2013-04) Beck, Sara; Turner, Jean; Lacy, John; Greathouse, Thomas K.; Lahad, Ohr; Lacy, John
The nearby dwarf galaxy II Zw 40 hosts an intense starburst. At the center of the starburst is a bright compact radio and infrared source, thought to be a giant dense H II region containing approximate to 14,000 O stars. Radio continuum images suggest that the compact source is actually a collection of several smaller emission regions. We accordingly use the kinematics of the ionized gas to probe the structure of the radio-infrared emission region. With TEXES on the NASA-IRTF we measured the 10.5 mu m [S IV] emission line with effective spectral resolutions, including thermal broadening, of similar to 25 and similar to 3 km s(-1) and spatial resolution similar to 1 ''. The line profile shows two distinct, spatially coextensive, emission features. The stronger feature is at galactic velocity and has FWHM 47 km s(-1). The second feature is similar to 44 km s(-1) redward of the first and has FWHM 32 km s(-1). We argue that these are two giant embedded clusters, and estimate their masses to be approximate to 3 x 10(5) M-circle dot and approximate to 1.5 x 10(5) M-circle dot. The velocity shift is unexpectedly large for such a small spatial offset. We suggest that it may arise in a previously undetected kinematic feature remaining from the violent merger that formed the galaxy.
Massive Star Formation Of The Sgr A East H II Regions Near The Galactic Center
(2010-12) Yusef-Zadeh, F.; Lacy, John H.; Wardle, M.; Whitney, B.; Bushouse, H.; Roberts, D. A.; Arendt, R. G.; Lacy, John H.
A group of four compact HII regions associated with the well-known 50 km s(-1) molecular cloud is the closest site of on-going star formation to the dynamical center of the Galaxy, at a projected distance of similar to 6 pc. We present a study of ionized gas based on the [Ne II] (12.8 mu m) line, as well as multi-frequency radio continuum, Hubble Space Telescope Pa alpha, and Spitzer Infrared Array Camera observations of the most compact member of the HII group, Sgr A East HII D. The radio continuum image at 6 cm shows that this source breaks up into two equally bright ionized features, D1 and D2. The spectral energy distribution of the D source is consistent with it being due to a 25 +/- 3M(circle dot) star with a luminosity of 8 +/- 3x10(4) L-circle dot. The inferred mass, effective temperature of the UV source, and the ionization rate are compatible with a young O9-B0 star. The ionized features D1 and D2 are considered to be ionized by UV radiation collimated by an accretion disk. We consider that the central massive star photoevaporates its circumstellar disk on a timescale of 3x10(4) years giving a mass flux similar to 3 x 10(-5) M-circle dot yr(-1) and producing the ionized material in D1 and D2 expanding in an inhomogeneous medium. The ionized gas kinematics, as traced by the [Ne II] emission, is difficult to interpret, but it could be explained by the interaction of a bipolar jet with surrounding gas along with what appears to be a conical wall of lower velocity gas. The other HII regions, Sgr A East A-C, have morphologies and kinematics that more closely resemble cometary flows seen in other compact HII regions, where gas moves along a paraboloidal surface formed by the interaction of a stellar wind with a molecular cloud.