3,259 research outputs found
The galactic center black hole as a possible retro-lens for the S2 orbiting star
Holz & Wheeler (\cite{hw}) have recently proposed that a Schwarzschild black
hole may act as a retro-lens which, if illuminated by a powerful light source,
deflects light ray paths to large bending angles and a series of luminous arcs
(or rings in the case of aligned objects) centered on the black hole may form.
Obviously, the most convenient geometry to get retro-lensing images would be
that of a very bright star close to a massive black hole, say the putative
M black hole at the galactic center. Recent
observations of the galactic center region in the -band have revealed the
presence of a very bright main sequence star (labelled S2) with mass
M orbiting at close distance (130-1900 AU) from Sgr A. The
relatively vicinity of S2 to the central massive black hole may offer a unique
laboratory to test the formation of retro-lensing images. The next generation
of space-based telescopes in the -band (like NGST) may have high enough
limiting magnitude necessary to observe such retro-lensing images.Comment: 4 pages, 2 Postscript figures, accepted for pubblications on
Astronomy and Astrophysic
The potential for high performance HgCdTe arrays at 4 microns
The potential of existing technology at Rockwell International in terms of the goals for astronomical detector arrays in the 3 to 5 micron interval is evaluated. Measurements have been obtained for a number of samples of HgCdTe diodes manufactured by Rockwell International. All the diodes reported on here had cutoff wavelengths at high temperatures of 4.6 to 4.7 microns. Although no confirming measurements were made, the cutoff wavelength is expected to move to 5 microns or beyond at the low temperatures of our tests. Diode sizes ranged from 20 to 150 microns. The test program yielded full diode curves and relative response at 3.4 microns for the sample diodes as a function of temperature. Dark currents are quoted below as the current passing through the diode with a back bias of 50 mV. The various diode types showed a wide range of behavior, both with regard to dark current and responsibility. The test results for one of the best diode types are illustrated. This detector has a size of 148 microns and a cutoff wavelength of 4.61 microns
Molecular Hydrogen and Paschen-alpha Emission in Cooling Flow Galaxies
We present near-infrared spectra obtained to search for Pa-alpha and
molecular hydrogen lines in edge-darkened (FR I-type) radio galaxies with
bright Halpha emission in the redshift range 0.0535<z<0.15. We find that all
three galaxies in our sample (PKS 0745-191, PKS 1346+26, & PKS2322-12) which
are associated with strong cooling flows also have strong Pa-alpha and H_2
(1-0) S(1) through S(5) emission, while other radio galaxies do not. Together
with earlier observations this confirms claims that cooling flow galaxies are
copious emitters of molecular hydrogen with large H_2 (1-0) S(3)/Pa-alpha
ratios in the range 0.5 to 2. The emission is centrally concentrated within the
inner few kiloparsec and could come from warm (T ~ 1000-1500 K) molecular
material which is being deposited by the cooling flow. We speculate that the
H_2 emission could be related to the interaction between the jets and this
molecular gas.Comment: ApJ Letters, in press, AAS LaTex, preprint also available at
http://www.astro.umd.edu/~hfalcke/publications.html#nirga
NICMOS Observations of Interaction Triggered Star Formation in the Luminous Infrared Galaxy NGC 6090
High resolution, 1.1, 1.6, and 2.2 micron imaging of the luminous infrared
galaxy NGC 6090 obtained with NICMOS of the Hubble Space Telescope are
presented. These new observations are centered on the two nuclei of the merger,
and reveal the spiral structure of the eastern galaxy and the amorphous nature
of the western galaxy. The nuclear separation of 3.2 kpc (H_0 = 75 km/s/Mpc)
indicates that NGC 6090 is at an intermediate stage of merging. Bright
knots/clusters are also visible in the region overlapping the merging galaxies;
four of these knots appear bluer than the underlying galaxies and have colors
consistent with young (<~ 10^7 yr) star clusters. The spatial coincidence of
the knots with the molecular gas in NGC 6090 indicates that much of the present
star formation is occuring outside of the nuclear region of merging galaxies,
consistent with recent studies of other double nuclei luminous infrared
galaxies.Comment: LaTex, 18 pages with 4 jpg figures, ApJ, in pres
The first 40 million years of circumstellar disk evolution: the signature of terrestrial planet formation
We characterize the first 40 Myr of evolution of circumstellar disks through
a unified study of the infrared properties of members of young clusters and
associations with ages from 2 Myr up to ~ 40 Myr: NGC 1333, NGC 1960, NGC 2232,
NGC 2244, NGC 2362, NGC 2547, IC 348, IC 2395, IC 4665, Chamaeleon I, Orion
OB1a and OB1b, Taurus, the \b{eta} Pictoris Moving Group, \r{ho} Ophiuchi, and
the associations of Argus, Carina, Columba, Scorpius-Centaurus, and
Tucana-Horologium. Our work features: 1.) a filtering technique to flag noisy
backgrounds, 2.) a method based on the probability distribution of deflections,
P(D), to obtain statistically valid photometry for faint sources, and 3.) use
of the evolutionary trend of transitional disks to constrain the overall
behavior of bright disks. We find that the fraction of disks three or more
times brighter than the stellar photospheres at 24 {\mu}m decays relatively
slowly initially and then much more rapidly by ~ 10 Myr. However, there is a
continuing component until ~ 35 Myr, probably due primarily to massive clouds
of debris generated in giant impacts during the oligarchic/chaotic growth
phases of terrestrial planets. If the contribution from primordial disks is
excluded, the evolution of the incidence of these oligarchic/chaotic debris
disks can be described empirically by a log-normal function with the peak at 12
- 20 Myr, including ~ 13 % of the original population, and with a post-peak
mean duration of 10 - 20 Myr.Comment: accepted for publication, the Astrophysical Journal (2017
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