Yusef-Zadeh F.,Northwestern University |
Wardle M.,Macquarie University |
Bushouse H.,STS Inc |
Dowell C.D.,Jet Propulsion Laboratory |
Roberts D.A.,Adler Planetarium and Astronomy Museum
Astrophysical Journal Letters | Year: 2010
We have investigated the nature of flare emission from Sgr A* during multi-wavelength observations of this source that took place in 2004, 2005, and 2006. We present evidence for dimming of submillimeter and radio flux during the peak of near-IR flares. This suggests that the variability of Sgr A* across its wavelength spectrum is phenomenologically related. The model explaining this new behavior of flare activity could be consistent with adiabatically cooling plasma blobs that are expanding but also partially eclipsing the background quiescent emission from Sgr A*.When a flare is launched, the plasma blob is most compact and is brightest in the optically thin regime whereas the emission in radio/submillimeter wavelengths has a higher opacity. Absorption in the observed light curve of Sgr A* at radio/submillimeter flux is due to the combined effects of lower brightness temperature of plasma blobs with respect to the quiescent brightness temperature and high opacity of plasma blobs. This implies that plasma blobs are mainly placed in the magnetosphere of a disk-like flow or further out in the flow. The depth of the absorption being larger in submillimeter than in radio wavelengths implies that the intrinsic size of the quiescent emission increases with increasing wavelength which is consistent with previous size measurements of Sgr A*. Lastly, we believe that occultation of the quiescent emission of Sgr A* at radio/submillimeter by IR flares can be used as a powerful tool to identify flare activity at its earliest phase of its evolution. © 2010. The American Astronomical Society. All rights reserved.
Yusef-Zadeh F.,Northwestern University |
Yusef-Zadeh F.,NASA |
Lacy J.H.,University of Texas at Austin |
Lacy J.H.,NASA |
And 5 more authors.
Astrophysical Journal | Year: 2010
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 ∼6 pc. We present a study of ionized gas based on the [Ne ii] (12.8 μm) line, as well as multi-frequency radio continuum, Hubble Space Telescope Paα, 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⊙ star with a luminosity of 8±3×104 L⊙. 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 3×104 years giving a mass flux ∼3 ±10-5 M⊙ 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.
Notis M.,Lehigh University |
Newbury B.,ExxonMobil |
Stephenson B.,Adler Planetarium and Astronomy Museum |
Stephenson G.B.,Argonne National Laboratory
Applied Physics A: Materials Science and Processing | Year: 2013
The astrolabe is an ancient analogue astronomical computing device used for calculations relating to position and time of the observer's location. In its most common form (the planispheric astrolabe), it consists of an engraved plate or series of plates held together and pinned in a housing, the assembly usually being made of brass. The present study describes the use of X-ray diffraction (XRD) and X-ray fluorescence (XRF) in a synchrotron to elucidate the composition of, and fabrication techniques used for, the major component parts of the astrolabe. The synchrotron XRF studies are compared to similar studies made with a handheld XRF instrument and the advantages and disadvantages of both approaches are discussed. © 2012 Springer-Verlag Berlin Heidelberg.
Masters K.L.,University of Portsmouth |
Nichol R.C.,University of Portsmouth |
Hoyle B.,University of Portsmouth |
Hoyle B.,University of Barcelona |
And 9 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2011
We present first results from Galaxy Zoo 2, the second phase of the highly successful Galaxy Zoo project. Using a volume-limited sample of 13665 disc galaxies (0.01 < z < 0.06 and Mr < -19.38), we study the fraction of galaxies with bars as a function of global galaxy properties like colour, luminosity and bulge prominence. Overall, 29.4 ± 0.5 per cent of galaxies in our sample have a bar, in excellent agreement with previous visually classified samples of galaxies (although this overall fraction is lower than that measured by automated bar-finding methods). We see a clear increase in the bar fraction with redder (g-r) colours, decreased luminosity and in galaxies with more prominent bulges, to the extent that over half of the red, bulge-dominated disc galaxies in our sample possess a bar. We see evidence for a colour bimodality for our sample of disc galaxies, with a 'red sequence' that is both bulge and bar dominated, and a 'blue cloud' which has little, or no, evidence for a (classical) bulge or bar. These results are consistent with similar trends for barred galaxies seen recently both locally and at higher redshift, and with early studies using the RC3. We discuss these results in the context of internal (secular) galaxy evolution scenarios and the possible links to the formation of bars and bulges in disc galaxies. © 2010 The Authors Monthly Notices of the Royal Astronomical Society © 2010 RAS.
O'Malia K.K.J.,University of Colorado at Boulder |
Thorburn J.A.,Carthage College |
Hammergren M.,Adler Planetarium and Astronomy Museum |
Snow T.P.,University of Colorado at Boulder |
And 3 more authors.
Astrophysical Journal | Year: 2010
We present the search for both diffuse interstellar bands (DIBs) and molecules in Comet 17P (Holmes) and Comet C/2007 W1 (Boattini) occultation observations. Absorption spectra were taken during stellar occultations by Comet Holmes of 31 and β Persei, and the occultation of BD+22 216 by Comet Boattini. While no signature of the comets was detected, we present upper limits for some common cometary molecules such as C2, C3, CH, CN and for the most common DIBs. We did not detect either comet in absorption, most likely because of the large distance between the line of sight to the star and the nucleus of the comet. Interstellar sight lines with comparable reddening to what was measured in Comet Holmes have DIB equivalent widths between 5 and 50 mÅ. However, future observations with closer approaches to a background star have great potential for spatially mapping molecule distributions in comets, and in discovering DIBs, if they are present, in comets. Future observations could detect DIBs and molecules if they are done: (1) less than ∼104-103 km from the nucleus (2) with a signal to noise in the background star of ∼300 and (3) with a resolving power of at least 38,000. © 2010. The American Astronomical Society. All rights reserved.