Showing papers by "Mark J. Reid published in 1997"

Radio Photospheres of Long-Period Variable Stars

Abstract: We report the detection of centimeter-wavelength emission from a sample of nearby long-period (Mira and semiregular) variables using the VLA. Six of the eight stars in the sample were detected. We find the continuum emission in the radio band to have a spectral index near 2.0, as expected for optically thick blackbody emission. The flux densities are a factor of ≈ 2 above the level expected from the optical photospheres of the stars. We monitored three stars over a period of nearly 2 yr and find flux density variations of less than ±15%. We partially resolved the stellar disk of W Hya and find an average diameter of 0080 ± 0015 and a brightness temperature of 1500 ± 570 K. Our observations suggest that long-period variables have a "radio photosphere" near 2R*, where R* is the stellar radius (defined by line-free regions of the optical spectrum). For the physical conditions expected in the radio photosphere, free electrons, obtained predominantly from the ionization of potassium and sodium, provide the dominant opacity through free-free interactions with neutral H and H2. A simple model with a single set of physical parameters can approximate all of our centimeter-wavelength data, as well as providing plausible sizes and brightness temperatures at far-IR wavelengths. At centimeter wavelengths, unity optical depth is achieved at a radius of about 4.8 × 1013 cm, where the density and temperature are ≈ 1.5 × 1012 cm-3 and ≈ 1630 K, respectively. The lack of variability of the centimeter-wavelength flux density for stars like o Ceti, R Leo, and W Hya limits variations of the temperature and/or radius of the radio photosphere to less than ±150 K and ±4 × 1012 cm. Also, any periodic shocks or disturbances near 2R* probably propagate outward at less than ≈ 5 km s-1 and/or are mostly damped. The radio photosphere lies just outside of a "molecular photosphere," seen optically in strong absorption lines of metallic oxides, and just inside of the SiO maser shell and the dust formation zone. Indeed, the inner boundary of the SiO maser emission region may be determined by continuum opacity in the radio photosphere. Our study suggests that the density and temperature in the SiO shell are ≈ 5 × 1010 cm-3 and ≈ 1300 K, respectively. Extrapolating our model outward to 1014 cm radius, where significant dust is detected at 10 μm wavelength and H2O masers are found, gives densities 3 × 109 cm-3 and temperatures 1100 K. Based on the sample of stars we observed, it appears that the flux density at radio frequencies from long-period variables can be modeled with a simple formula. Given the excellent agreement between measured and modeled flux densities, it is possible that distances can be estimated from a flux density measurement with a precision of about 10%. Since the radio-frequency emission from long-period variables has a well-defined spectrum, is very compact, and is relatively constant in time, we suggest these stars can be used to determine the absolute flux density scale for millimeter- and submillimeter-wavelength interferometers.

The Position of Sagittarius A*: Accurate Alignment of the Radio and Infrared Reference Frames at the Galactic Center

Abstract: We present a novel approach to the long-standing problem of locating the position of the compact nonthermal radio source Sgr A* on infrared images of the Galactic center region. Using the Very Large Array, we have detected SiO and H2O maser emission toward several sources within the central parsec of our Galaxy. These masers arise from the innermost parts of circumstellar envelopes of giant and supergiant stars that are members of the nuclear star cluster and appear as compact infrared sources in a diffraction-limited 2.2 μm infrared image. One of the SiO masers is associated with the M-type supergiant IRS 7, the most prominent 2.2 μm point source in the Galactic center region. The radio data allow measurements of the maser positions relative to the compact nonthermal radio continuum source Sgr A* with milliarcsecond accuracy. Because stellar SiO masers near the Galactic center trace their host stars to within a few milliarcseconds, these relative positions can be used to calibrate the plate scale and rotation of the infrared image. Our method allows registration of the radio relative to the infrared reference frame with an estimated accuracy of 003. Using the improved position accuracy we put a stringent upper limit on the 2.2 μm flux density of Sgr A* that is significantly lower than values predicted by recent theoretical model calculations.

The Proper Motion of SGR A

Abstract: We have now been measuring the position of Sgr A*, the candidate supermassive black hole at the center of the Galaxy, with the VLBA for about 8 years. Sgr A* appears to move almost entirely along the Galactic Plane at a rate of 6.37±0.02 mas yr. For a distance to the Galactic Center of 8.0±0.5 kpc, this translates to 241 ± 15 km s , consistent with that expected for a stationary object observed from the Sun as it orbits the Galactic Center. The motion of Sgr A* out of the plane of the Galaxy, after removing the 7 km s motion of the Sun in that direction, is less than about 2 km s . Combining stellar orbital information (measured in the infrared) with the upper limit of 2 km s for the intrinsic proper motion of Sgr A* (perpendicular to the Galactic plane), places a lower limit on the mass of Sgr A* of 2 × 10 M⊙. Thus, most of the mass sensed by stellar orbits is tied to the compact radio source Sgr A*, whose size is less than 1 AU, yielding the strongest case ever for a SMBH. This also argues against “exotic” forms of mass, postulated to explain the extreme mass concentration at the Galactic Center.

Shocks in the Radio Photospheres of Long Period Variable Stars

Abstract: At a distance of a few stellar radii and beyond from the center of a Mira variable, dust condenses (see Danchi et al. 1994) and mass begins to stream away from the star as a result of radiation pressure on the dust and subsequent drag on molecules. Since a significant fraction of the mass returned to the interstellar medium comes from red giant stars, this is an important process which affects star formation and galaxy evolution. However, fundamental questions still remain relating to the movement of mass outwards to a few stellar radii and the conditions under which dust forms. Thus, new approaches to study the region between the optical photosphere and the dust formation zone are extremely valuable.