arXiv subjects

M. J. Reid

Publications and source records attributed to M. J. Reid.

130 records · Page 8Linked to original sources

arXiv2001

How Mira Variables Change Visual Light by a Thousand-fold

Mira variables change visual light by up to 8 magnitudes over their roughly yearly cycle. Here we present a simple explanation for the extremely large amplitudes of light curves of oxygen-rich Mira variables. Metallic oxides, such as TiO, form throughout the stellar atmosphere as the star cools when approaching minimum light. When this happens, the visual light can be almost completely absorbed at large radii, extending the visual photosphere to nearly twice its nominal size. At these large radii, temperatures can fall to approximately 1400 K and essentially all of the star's radiation emerges in the infrared. Since almost no optical light is emitted at these low temperatures, Mira variables can decrease their visual light by more than a thousand-fold and almost "disappear" to the human eye.

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arXiv1999

The Proper Motion of SgrA*: I. First VLBA Results

We observed Sgr A* and two extragalactic radio sources nearby in angle with the VLBA over a period of two years and measured relative positions with an accuracy approaching 0.1 mas. The apparent proper motion of Sgr A* relative to J1745-283 is 5.90 +/- 0.4 mas/yr, almost entirely in the plane of the Galaxy. The effects of the orbit of the Sun around the Galactic Center can account for this motion, and any residual proper motion of Sgr A*, with respect to extragalactic sources, is less than about 20 km/s. Assuming that Sgr A* is at rest at the center of the Galaxy, we estimate that the circular rotation speed in the Galaxy at the position of the Sun is 219 +/- 20 km/s, scaled by Ro/8.0 kpc. Current observations are consistent with Sgr A* containing all of the nearly 2.6 x 10^6 solar masses, deduced from stellar proper motions, in the form of a massive black hole. While the low luminosity of Sgr A*, for example, might possibly have come from a contact binary containing of order 10 solar masses, the lack of substantial motion rules out a "stellar" origin for Sgr A*. The very slow speed of Sgr A* yields a lower limit to the mass of Sgr A* of about 1,000 solar masses. Even for this mass, Sgr A* appears to be radiating at less than 0.1 percent of its Eddington limit.

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arXiv1998

Imaging the z=0.9 Absorbing Cloud toward 1830-211

We present spectroscopic imaging observations of the molecular and HI 21cm absorbing cloud at z = 0.885 toward the `Einstein ring' radio source PKS 1830-211. We derive a cloud size between 10 h^{-1} pc and 600 h^{-1} pc, and M(H_2) > 2.6x10^{4} h^{-2} Msolar. The temperature of the ambient radiation field is 4.5+/-0.9 K, consistent with the microwave background temperature at z = 0.885. The velocity difference for absorption on opposite sides of the ring is -146 km s^{-1}, which is consistent with the galaxian rotation velocity derived from gravitational lens models.

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arXiv1997

Redshifted Neutral Hydrogen 21cm Absorption toward Red Quasars

We have searched for redshifted neutral hydrogen 21cm absorption toward sources from the Stickel et al. `red quasar' sub-sample. Five of these red quasars have been searched for redshifted HI 21cm absorption to optical depth levels of a few percent, and four show strong absorption. This 80% success rate for the red quasars compares to the much lower success rate of only 11% for detecting HI 21cm absorption associated with optically selected Mg II absorption line systems. The large neutral hydrogen column densities seen toward the red quasars provide circumstantial evidence supporting the dust reddening hypothesis, as opposed to an intrinsically red spectrum for the AGN emission mechanism. The data on the red quasar sub-sample support the models of Fall and Pei for dust obscuration by damped Ly alpha absorption line systems and suggest that: (i) there may be a significant, but not dominant, population of quasars missing from optically selected samples due to dust obscuration, perhaps as high as 20% at the POSS limit for an optical sample with a redshift distribution similar to the 1 Jy, flat spectrum quasar sample, and (ii) optically selected samples may miss about half the high column density quasar absorption line systems. The redshifted HI 21cm absorption line detections are toward the sources: 0108+388 at z = 0.6685, 0500+019 at z = 0.5846, and 1504+377 at z = 0.6733. No absorption is seen toward 2149+056 at z = 0.740 at a level below that seen for the three detections. In some systems the absorbing gas is in the vicinity of the AGN, either circumnuclear material or material in the general ISM of the AGN's host galaxy, and in other systems the absorption is by gas associated with galaxies cosmologically distributed along the line of sight to the quasar.

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