arXiv subjects

I. N. Reid

Publications and source records attributed to I. N. Reid.

45 records · Page 3Linked to original sources

arXiv1998

M Subdwarfs: The Population II Luminosity Function

We present results of a study of very low mass halo stars. Using a sample of proper motion stars identified from plate material taken as part of the first and second Palomar Sky Surveys, we measure the space density, metallicity distribution, and kinematics of the Population II M subdwarfs. Our overall luminosity function is in good agreement with previous analyses of the space density of nearby very-low-mass halo subdwarfs, and confirms the discrepancy between local analyses and the space densities inferred from deep HST starcounts. We show for the first time that both the metallicity distribution and kinematics of late-type halo subdwarfs are consistent with those of their higher mass metal-poor counterparts. Dividing our sample by abundance, we find no evidence that the mass function of field halo stars is dependent upon metallicity. We provide data for three nearby subdwarfs that may merit additional observations.

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arXiv1998

High-resolution Spectra of Very Low-Mass Stars

We present the results of high-resolution (1-0.4A) optical spectroscopy of a sample of very low-mass stars. These data are used to examine the kinematics of the stars at the bottom of the hydrogen-burning main sequence. No evidence is found for a significant difference between the kinematics of the stars in our sample with I-K > 3.5 (MBol > 12.8) and those of more massive M-dwarfs (MBol = 7-10). A spectral atlas at high (0.4A) resolution for M8-M9+ stars is provided, and the equivalent widths of CsI, RbI and Halpha lines present in our spectra are examined. We analyse our data to search for the presence of rapid rotation, and find that the brown dwarf LP 944-20 is a member of the class of ``inactive, rapid rotators''. Such objects seem to be common at and below the hydrogen burning main sequence. It seems that in low-mass/low-temperature dwarf objects either the mechanism which heats the chromosphere, or the mechanism which generates magnetic fields, is greatly suppressed.

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arXiv1998

The Multiplicity of the Hyades and its Implications for Binary Star Formation and Evolution

A 2.2 micron speckle imaging survey of 167 bright (K $< 8.5$ mag) Hyades members reveals a total of 33 binaries with separations spanning 0."044 to 1."34 and magnitude differences as large as 5.5 mag. Of these binaries, 13 are new detections. Combining the results of this survey with previous radial velocity, optical speckle, and direct imaging Hyades surveys, the detected multiplicity of the sample is: 98 singles, 59 binaries, and 10 triples. A statistical analysis of this sample investigates a variety of multiple star formation and evolution theories. Accounting for the inability to detect high flux ratio binaries results in an implied companion star fraction (csf) of 0.30 +/- 0.06 in this separation range 0."1 to 1."07 (5 to 50 AU). The Hyades csf is intermediate between the values derived from observations of T Tauri stars (T Tauri csf=0.40 +/- 0.08) and solar neighborhood G-dwarfs (G-dwarf csf=0.14 +/- 0.03). This result allows for an evolution of the csf from an initially high value for the pre-main sequence to that found for main sequence stars. Within the Hyades, the csf is independent of the radial distance from the cluster center and the primary star mass. The distribution of mass ratios is best fit by a power law q^(-1.3 +/- 0.3) and shows no dependence on the primary mass, binary separation, or the radial distance from the cluster center. Overall, the Hyades data are consistent with scale-free fragmentation, but inconsistent with capture in small clusters and disk-assisted capture in small clusters. Without testable predictions, scale-dependent fragmentation and disk fragmentation cannot be assessed with the Hyades data.

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arXiv1997

Masses and Gravities of Blue Horizontal Branch Stars Revisited

Previous spectroscopic analyses of Blue Horizontal Branch (BHB) stars in six globular clusters revealed too low masses in four clusters when compared to canonical evolutionary theory, while the masses of the BHB stars in NGC 6752 and M 5 are found to be consistent with theory. We recalculated BHB star masses using new cluster distances derived by Reid (1997a,b) from HIPPARCOS parallaxes of local subdwarfs by main sequence fitting. The new distances are larger than previous estimates resulting in larger masses for the BHB stars. Since the increase in distance is small for NGC 6752 and M 5, the agreement with predicted masses persists. For M 15 and M 92 the masses now come into good agreement with theoretical predictions, while for NGC 288 and NGC 6397 the mass deficit is reduced but the BHB star masses remain slightly too low. Previous spectroscopic analyses also highlighted the problem of too low gravities for some BHB stars. The gravities and absolute magnitudes of BHB stars are revisited in the light of new evolutionary Horizontal Branch models.

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arXiv1997

MACHO Alert 95-30 : First Real-Time Observation of Extended Source Effects in Gravitational Microlensing

We present analysis of MACHO Alert 95-30, a dramatic gravitational microlensing event towards the Galactic bulge whose peak magnification departs significantly from the standard point-source microlensing model. Alert 95-30 was observed in real-time by the Global Microlensing Alert Network (GMAN), which obtained densely sampled photometric and spectroscopic data throughout the event. We interpret the light-curve ``fine structure'' as indicating transit of the lens across the extended face of the source star. This signifies resolution of a star several kpc distant. We find a lens angular impact parameter theta_{min}/theta_{source} = 0.715 +/- 0.003. This information, along with the radius and distance of the source, provides an additional constraint on the lensing system. Spectroscopic and photometric data indicate the source is an M4 III star of radius 61 +/- 12 Rsun, located on the far side of the bulge at 9 kpc. We derive a lens angular velocity, relative to the source, of 21.5 +/- 4.9 km/s/kpc, where the error is dominated by uncertainty in the source radius. Likelihood analysis yields a median lens mass of 0.67{+2.53}{-0.46} Msun, located with 80% probability in the Galactic bulge at a distance of 6.93{+1.56}{-2.25} kpc. If the lens is a main-sequence star, we can include constraints on the lens luminosity. This modifies our estimates to M_lens = 0.53{+0.52}{-0.35} Msun and D_lens = 6.57{+0.99}{-2.25} kpc. Spectra taken during the event show that the absorption line equivalent widths of H alpha and the TiO bands near 6700 A vary, as predicted for microlensing of an extended source. This is most likely due to center-to-limb variation in the stellar spectral lines. These data demonstrate the feasibility of using microlensing limb crossings as a tool to probe stellar atmospheres directly.

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arXiv1997

Faint M-dwarfs and the structure of the Galactic disk

We use broadband photometry and low-resolution spectra of a complete sample of late-K and M dwarfs brighter than I=22 in three fields at high galactic latitude to study issues relating to galactic structure and large scale abundance gradients in the Galaxy. The observed starcounts in each field are a good match to the predictions of models based on deep starcount data in other intermediate-latitude fields, and these models identify the late-type stars as members of the Galactic disk. Abundances for these late type stars are estimated via narrowband indices that measure the strength of the TiO and CaH bands in their spectra. Our results show that the average abundance in the Galactic disk remains close to solar even at heights of more than 2 kpc above the Plane.

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arXiv1996

Starcounts Redivivus II: Deep Starcounts with Keck and HST and the Luminosity Function of the Galactic Halo

We have combined deep starcount data with Galaxy model predictions to investigate how effectively such measurements probe the faint end of the halo luminosity function. We have tested a number of star/galaxy classification techniques using images taken in 0.5 arcsecond seeing with LRIS on the Keck telescope, and we find that different combinations of these techniques can produce variations of 10 \% in the inferred starcounts at R=22.5 and 30 \% at R=24.5 magnitudes. The decreasing average angular size of galaxies with fainter magnitude effectively limits ground-based work to R $<$ 25.5 magnitudes. The higher angular resolution provided by HST allows one to probe at least 2 magnitudes fainter, but the small field-size is a significant limitation. In either case, our models show that the contribution from halo subdwarfs is effectively limited to colours of (R-I) $<$ 1.0, with the redder stars being members of the Galactic disk. The apparent increase in number density for M$_V > 10$ in the derived luminosity function is a result of contributions from disk stars at fainter absolute magnitudes and does not provide evidence for an upturn in the halo subdwarf mass function. Indeed, starcount data alone are {\it not} an effective method of probing the shape of the halo luminosity function close to the hydrogen-burning limit. Finally, we examine how the Hubble Deep Field observations can be used to constrain the contribution of various stellar components to the dark-matter halo.

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arXiv1995

The Distance to IC 10 from Near-Infrared Observations of Cepheids

We have measured the distance to the Local Group dwarf irregular galaxy IC 10 from near-infrared $JHK$ observations of Cepheids. The reddening-corrected distance modulus is $24.57\pm0.21$ mag or $0.82\pm 0.08$ Mpc. This distance lies at the short end of the range of previous estimates, and is significantly more accurate than all prior determinations. At this distance, IC 10 is comparable in total mass to the Small Magellanic Cloud, but has much more vigorous star formation.

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arXiv1994

Optical and Infrared Observations of SGR 1806-20

The soft gamma-ray repeater (SGR) 1806$-$20 is associated with the center-brightened non-thermal nebula G~10.0$-$0.3, thought to be a plerion. As in other plerions, a steady \Xray\ source, AX~1805.7$-$2025, has been detected coincident with the peak of the nebular radio emission. Vasisht et al.\ have shown that the radio peak has a core-jet appearance, and argue that the core marks the true position of the SGR. At optical wavelengths, we detect three objects in the vicinity of the radio core. Only for the star closest to the core, barely visible in the optical but bright in the infrared ($K=8.4\,$mag.), the reddening is consistent with the high extinction ($A_V\simeq30\,$mag.) that has been inferred for AX~1805.7$-$2025. From the absence of CO band absorption, we infer that the spectral type of this star is earlier than late~G/early~K. The large extinction probably arises in a molecular cloud located at a distance of 6$\,$kpc, which means that the star, just like AX~1805.7$-$2025, is in or behind this cloud. This implies that the star is a supergiant. Since supergiants are rare, a chance coincidence with the compact radio core is very unlikely. To our knowledge, there are only three other examples of luminous stars embedded in non-thermal radio nebulae, SS~433, \mbox{Cir X-1} and G~70.7+1.2. Given this and the low coincidence probability, we suggest that the bright star is physically associated with SGR~1806$-$20, making it the first stellar identification of a high-energy transient.

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