Smithsonian Astrophysical Observatory

About: Smithsonian Astrophysical Observatory is a facility organization based out in Cambridge, Massachusetts, United States. It is known for research contribution in the topics: Galaxy & Stars. The organization has 1665 authors who have published 3622 publications receiving 132183 citations. The organization is also known as: SAO.

Oscillator Strengths for High-level Transitions in Hydrogen

Abstract: TRANSITIONS between high quantum levels of atomic hydrogen can produce spectral lines in the radio region. Observers have detected a number of such emissions from hydrogen gas in various radio sources. Theoretical interpretation of such observations requires knowledge of the oscillator strengths fn+c,n for transitions described by the quantum transition n+c→n. For n large and c small, the fs follow the asymptotic formula with Jc(c) is the Bessel function of equal order and argument and J′c(c) is the first derivative with respect to the argument. Table 1 contains a few representative values of M(c).

Active Star Formation toward the Ultracompact H II Regions G45.12+0.13 and G45.07+0.13

Abstract: A multiwavelength study of the molecular cores containing the ultracompact (UC) H II regions G45.12+0.13 and G45.07+0.13 reveals a series of phenomenological differences that distinguish the age of these cores in terms of their development of high-mass star formation. First, we report the discovery of massive, bipolar molecular outflows from both UC H II regions. The G45.12+0.13 UC H II region lies centered on a spatially extended, 6 km s-1 outflow that we have mapped in the CO J = 2 ? 1, 3 ? 2, 6 ? 5,13CO 2 ? 1, and C18O 2 ? 1 transitions at the Caltech Submillimeter Observatory (CSO). The broad bipolar structure is optically thick in the 12CO line. The 13CO measurements imply a large outflow mass of 4800 M? (12% of the total cloud mass). Interferometric observations with the Owens Valley Radio Observatory (OVRO) millimeter array in the 13CO 1 ? 0 line resolve the gas into at least two outflows, one of which emanates from the 4.0 Jy, 110 GHz source identified with the UC H II region. An additional outflow is driven by an adjacent young, embedded object that contributes to the extended submillimeter continuum emission imaged with the CSO bolometer array camera. Lying in a separate core a few arcminutes away, the G45.07+0.13 UC H II region contains H2O masers and presents higher velocity (11 km s-1) yet more compact CO emission. An outflow has been detected in the CO 6 ? 5 transition, along with a compact submillimeter continuum source. OVRO observations in the CS J = 2 ? 1 transition confirm a compact outflow centered on the 98 GHz continuum source toward which infall is also seen in the form of redshifted absorption. The multiple outflows, higher CO antenna temperatures, more extended submillimeter and radio continuum emission, and lack of H2O masers all distinguish the core containing G45.12+0.13 as a more advanced site of massive star formation than the neighboring core containing G45.07+0.13.

MMT extremely metal-poor galaxy survey. I. An efficient technique for identifying metal-poor galaxies

Abstract: We demonstrate a successful strategy for identifying extremely metal-poor galaxies. Our preliminary survey of 24 candidates contains 10 metal-poor galaxies of which 4 have , some of the lowest-metallicity blue compact galaxies known to date. Interestingly, our sample of metal-poor galaxies have systematically lower metallicity for their luminosity than comparable samples of blue compact galaxies, dwarf irregulars, and normal star-forming galaxies. Our metal-poor galaxies share very similar properties, however, with the host galaxies of nearby long-duration gamma-ray bursts (GRBs), including similar metallicity, stellar ages, and star formation rates. We use Hβ to measure the number of OB stars present in our galaxies and estimate a core-collapse supernova rate of ~10−3 yr−1. A larger sample of metal-poor galaxies may provide new clues about the environment where GRBs form and may provide a list of potential GRB hosts.

Reconnaissance Geology of the Amaro Horst, Southern Ethiopian Rift

Abstract: The Amaro horst is a 90-km by 25-km block of sialic crust uplifted 1.5 km above the axis of the main Ethiopian rift, near its southern termination. The Amaro horst resembles the famous Ruwenzori horst of the Western rift, but additionally the former has had a Neogene-Quaternary volcanic history that reveals episodic uplift of the horst from the axis of a bilaterally downwarping-downtilting rift floor. Vertical forces that produced normal faults uplifted the horst to stratigraphic elevations higher than the plateaus outside the rift.

Polaris: Amplitude, Period Change, and Companions

Abstract: Polaris has presented us with the rare phenomenon of a Cepheid with a pulsation amplitude that has decreased over the last 50 yr. In this study we have used this property to see whether the amplitude decrease during the last 15 yr has had any effect on upper atmosphere heating. We obtained IUE high- and low-resolution spectra but found no change in either the Mg II chromospheric emission or the flux at 1800 A between 1978 and 1993 when the pulsation amplitude dropped by 50% (from 2.8 to 1.6 km s-1). The energy distribution from 1700 A through V, B, R(KC), and I(KC) is like that of a nonvariable supergiant of the same color rather than a full amplitude Cepheid in that it has more flux at 1800 A than the full amplitude Cepheid δ Cep. Polaris also has a rapidly changing period (3.2 s yr-1), in common with other overtone pulsators. We argue that this is a natural consequence of the different envelope locations that dominate pulsation growth rates in fundamental and overtone pulsation. In fundamental mode pulsators, the deeper envelope is more important in determining growth rates than for overtone pulsators. For fundamental mode pulsators, evolutionary changes in the radius produce approximately linear changes in period. In overtone pulsators, pulsation reacts to small evolutionary changes in a more unstable way because the modes are more sensitive to high envelope features such as opacity bumps, and the growth rates for the many closely spaced overtone modes change easily. Finally, the upper limit to the X-ray flux from an Einstein observation implies that the companion in the astrometric orbit is earlier than F4 V. The combination of upper and lower limits on the companion from IUE and Einstein respectively catch the companion mass between 1.7 and 1.4 M☉. The X-ray limit is consistent with the more distant companion α UMi B being a physical companion in a hierarchal triple system. However the X-ray limits require that the even more distant companions α UMi C and D are too old to be physically associated with Polaris.