arXiv subjects

John C. Martin

Publications and source records attributed to John C. Martin.

23 records · Page 2Linked to original sources

arXiv2010

Mid-Cycle Changes in Eta Carinae

In late 2006, ground-based photometry of $η$ Car plus the Homunculus showed an unexpected decrease in its integrated apparent brightness, an apparent reversal of its long-term brightening. Subsequent HST/WFPC2 photometry of the central star in the near-UV showed that this was not a simple reversal. This multi-wavelength photometry did not support increased extinction by dust as the explanation for the decrease in brightness. A spectrum obtained with GMOS on the Gemini-South telescope, revealed subtle changes mid-way in $η$ Car's 5.5 yr spectroscopic cycle 0when compared with HST/STIS spectra at the same phase in the cycle. At mid-cycle the secondary star is 20--30 AU from the primary. We suggest that the spectroscopic changes are consistent with fluctuations in the density and velocity of the primary star's wind, unrelated to the 5.5 yr cycle but possibly related to its latitude-dependent morphology. We also discuss subtle effects that must be taken into account when comparing ground-based and HST/STIS spectra.

astro-ph.SR↗

arXiv2007

Halo Star Streams in the Solar Neighborhood

We have assembled a sample of halo stars in the solar neighborhood to look for halo substructure in velocity and angular momentum space. Our sample includes red giants, RR Lyrae, and red horizontal branch stars within 2.5 kpc of the Sun with [Fe/H] less than -1.0. It was chosen to include stars with accurate distances, space velocities, and metallicities as well as well-quantified errors. We confirm the existence of the streams found by Helmi and coworkers, which we refer to as the H99 streams. These streams have a double-peaked velocity distribution in the z direction. We use the results of modeling of the H99 streams by Helmi and collaborators to test how one might use v_z velocity information and radial velocity information to detect kinematic substructure in the halo. We find that detecting the H99 streams with radial velocities alone would require a large sample. We use the velocity distribution of the H99 streams to estimate their age. From our model of the progenitor of the H99 streams, we determine that it was accreted between 6 and 9 Gyr ago. The H99 streams have [alpha/Fe] abundances similar to other halo stars in the solar neighborhood, suggesting that the gas that formed these stars were enriched mostly by Type II SNe. We have also discovered in angular momentum space two other possible substructures, which we refer to as the retrograde and prograde outliers. The retrograde outliers are likely to be halo substructure, but the prograde outliers are most likely part of the smooth halo. The retrograde outliers have significant structure in the v_phi direction and show a range of [alpha/Fe]. The methods presented in this paper can be used to exploit the kinematic information present in future large databases like RAVE, SDSSII/SEGUE, and Gaia.

astro-ph↗

arXiv2002

The Masses Of The B-Stars In The High Galactic Latitude Eclipsing Binary IT Lib

A number of blue stars which appear to be similar to Population I B-stars in the star forming regions of the galactic disk are found more than 1 kpc from the galactic plane. Uncertainties about the true distances and masses of these high latitude B-stars has fueled a debate as to their origin and evolutionary status. The eclipsing binary IT Lib is composed of two B-stars, is approximately one kiloparsec above the galactic plane, and is moving back toward the plane. Observations of the light and velocity curves presented here lead to the conclusion that the B-stars in this system are massive young main-sequence stars. While there are several possible explanations, it appears most plausible that the IT Lib system formed in the disk about 30 million years ago and was ejected on a trajectory taking it to its present position.

astro-ph↗

arXiv1999

A Re-interpretation of Historical References to the Supernova of 1054 AD

In this paper we re-examine historical references to the supernova event of 1054 AD with a view to establishing a sequence astronomical events which minimizes apparent conflicts between the various sources. We find that the explosion of the supernova is likely to have occurred weeks to months before the commonly accepted date of July 4th 1054. This view is strongly supported by a number of European references to events visible in the evening sky during the spring which are likely to be associated with the appearance of the supernova. We find the best fit to the light curve based on Chinese observations and a maximum visible apparent magnitude for a supernova located at the distance to the Crab Nebula also confirms the earlier explosion date.

astro-ph↗

arXiv1998

A New Analysis of RR Lyrae Kinematics in the Solar Neighborhood

Full space velocities are computed for a sample of 130 nearby RR Lyrae variables using both ground-based and Hipparcos proper motions. In many cases proper motions from multiple sources have been averaged to produce a significant improvement in the transverse space velocity errors. In most cases, this exceeds the accuracy of Hipparcos proper motions alone. The velocity ellipsoids computed for halo and thick disk samples are in agreement with previous studies. No distinct sample of thin disk RR Lyraes has been isolated but there is kinematic evidence for some thin disk stars in our thick disk samples. A sample of 21 stars with [Fe/H] < -1.0 and disk-like kinematics have been isolated. It is concluded from their kinematics and spatial distribution that these stars represent a sample of RR Lyraes in the metal weak tail of the thick disk. In the halo samples the distribution of V velocities is not gaussian, even when the metal weak thick disk stars are removed. Also, a plot of U and W velocities as a function of V velocity for the kinematically unbiased halo sample shows some curious structure. The cause of these kinematic anomalies is not clear. In addition, systematic changes to the distance scale within the range of currently accepted values of Mv(RR) are shown to significantly change the calculated halo kinematics. Fainter values of Mv(RR), such as those obtained by statistical parallax (~ 0.60 to 0.70 at [Fe/H]=-1.9), result in local halo kinematics similar to those reported in independent studies of halo kinematics, while brighter values of Mv(RR), such as those obtained through recent analysis of Hipparcos subdwarf parallaxes (~ 0.30 to 0.40 at [Fe/H]=-1.9), result in a halo with retrograde rotation and significantly enlarged velocity dispersions.

astro-ph↗