Showing papers by "M. H. van Kerkwijk published in 1998"

The Spectrum of the Brown Dwarf Gliese 229B

Abstract: We present a spectrum of the cool K) brown dwarf Gliese 229B. This spectrum, with a (T eff \ 900 relatively high signal-to-noise ratio per spectral resolution element spans the wavelength range (Z30), from 0.837 k mt o 5.0 km. We identify a total of four di†erent major methane absorption features, includ- ing the fundamental band at 3.3 km, at least four steam bands, and two neutral cesium features. We con-rm the recent detection of carbon monoxide (CO) in excess of what is predicted by thermochemical equilibrium calculations. Carbon is primarily involved in a chemical balance between methane and CO at the temperatures and pressures present in the outer parts of a brown dwarf. At lower temperatures, the balance favors methane, while in the deeper, hotter regions, the reaction reverses to convert methane into CO. The presence of CO in the observable part of the atmosphere is therefore a sensitive indicator of vertical Nows. The high signal-to-noise ratio in the 1 k mt o 2.5 km region permits us to place con- straints on the quantity of dust in the atmosphere of the brown dwarf. We are unable to reconcile the observed spectrum with synthetic spectra that include the presences of dust. The presence of CO but lack of dust may be a clue to the location of the boundaries of the outer convective region of the atmo- sphere: The lack of dust may mean that it is not being conveyed into the photosphere by convection, or that it exists in patchy clouds. If the dust is not in clouds, but rather sits below the outer convective region, we estimate that the boundary between outer convective and inner radiative layers is between 1250 K and 1600 K, in agreement with recent models. Subject headings: circumstellar matter E line: identi-cation E stars: individual (Gliese 229B) E stars: low-mass, brown dwarfs

Optical Observations of the Isolated Neutron Star RX J0720.4-3125

Abstract: RX J0720.423125 is an unidentified bright, soft X-ray source that shows pulsations at an 8.39 s period and has a thermal spectrum. We present deep B- and R-band images of its X-ray localization. We find one possible counterpart in the X-ray error box, with magnitudes and . The very high B 5 26.6 5 0.2 R 5 26.9 5 0.3 X-ray‐to‐optical flux ratio confirms that this object is an isolated neutron star. We discuss possible models, and we conclude that only two are consistent with the data and at the same time are able to draw from a large enough population to make finding one nearby likely. In our opinion, the second criterion provides a stringent constraint but appears to have been ignored so far. The first model, suggested earlier, is that RX J0720.423125 is a weakly magnetized neutron star accreting from the interstellar medium. The second is that it is a relatively young, highly magnetized neutron star, a “magnetar,” which is kept hot by magnetic field decay.

Warped disks as a possible origin of torque reversals in accretion-powered pulsars

Abstract: Enigmatic transitions between spin-up and spin-down have been observed in several X-ray pulsars accreting matter via an accretion disk In these transitions, the torque changes sign but remains at nearly the same magnitude It has been noted previously that alternating prograde and retrograde disk flows would explain many features of the torque reversals, although it has been unclear how a stable retrograde disk could be formed We suggest that the reversals may be related to the disk at times being warped to such an extent that the inner region becomes tilted by more than 907 This region would thus become retrograde, leading to a negative torque Accretion disk models can show such behavior, if account is taken of a warping instability due to irradiation The resulting “flipovers” of the inner parts of the disk can reproduce most characteristics of the observations, although it remains unclear what sets the timescale on which the phenomenon occurs If this model were correct, it would have a number of ramifications, for instance, that in the spin-down state, the X-ray source would mostly be observed through the accretion disk Subject headings: accretion, accretion disks — binaries: close — pulsars: individual (Centaurus X-3, GX 114, OAO 16572415, 4U 1626267) — stars: neutron — X-rays: stars

The Spectrum of the Brown Dwarf Gliese 229B

Abstract: We present a spectrum of the cool (T_eff = 900 K) brown dwarf Gliese 229B. This spectrum, with a relatively high signal-to-noise ratio per spectral resolution element (> 30), spans the wavelength range from 0.837 microns to 5.0 microns. We identify a total of four different major methane absorption features, including the fundamental band at 3.3 microns, at least four steam bands, and two neutral cesium features. We confirm the recent detection of carbon monoxide (CO) in excess of what is predicted by thermochemical equilibrium calculations. Carbon is primarily involved in a chemical balance between methane and CO at the temperatures and pressures present in the outer parts of a brown dwarf. At lower temperatures, the balance favors methane, while in the deeper, hotter regions, the reaction reverses to convert methane into CO. The presence of CO in the observable part of the atmosphere is therefore a sensitive indicator of vertical flows. The high signal-to-noise ratio in the 1 to 2.5 microns region permits us to place constraints on the quantity of dust in the atmosphere of the brown dwarf. We are unable to reconcile the observed spectrum with synthetic spectra that include the presences of dust. The presence of CO but lack of dust may be a clue to the location of the boundaries of the outer convective region of the atmosphere: The lack of dust may mean that it is not being conveyed into the photosphere by convection, or that it exists in patchy clouds. If the dust is not in clouds, but rather sits below the outer convective region, we estimate that the boundary between outer convective and inner radiative layers is between 1250 K and 1600 K, in agreement with recent models.

Phase-resolved spectroscopy of the low-mass X-ray binaries 1636-536/V801 Arae and 1735-444/V926 Scorpii

Abstract: We present phase-resolved spectroscopy of the op- tical counterparts V 801 Ara and V 926 Sco of the low-mass X-ray binaries 1636-536 and 1735-444. To assist in the in- terpretation of the spectroscopic observations we derive new ephemerides for the photometric variations of both sources. Su- perior conjunction of the line emission region occurs at photo- metric phase 0.7 in both sources, which indicates that these variations are dominated by a component originating from the point where the mass stream from the donor star intersects the outer disk. We nd that the properties of 1636-536/V 801 Ara and 1735-444/V 926 Sco are very similar.

Optical Observations of the Isolated Neutron Star RX J0720.4-3125

Abstract: RX J0720.4-3125 is an unidentified bright soft X-ray source which shows pulsations at a 8.39 s period and has a thermal spectrum. We present deep B and R band images of its X-ray localization. We find one possible counterpart in the X-ray error box, with magnitudes B=26.6+/-0.2 and R=26.9+/-0.3. The very high X-ray to optical flux ratio confirms that this object is an isolated neutron star. We discuss possible models and conclude that only two are consistent with the data and at the same time are able to draw from a large enough population to make finding one nearby likely. In our opinion the second criterion provides a stringent constraint but appears to have been ignored so far. The first model, suggested earlier, is that RX J0720.4-3125 is a weakly magnetized neutron star accreting from the interstellar medium. The second is that it is a relatively young, highly magnetized neutron star, a ``magnetar'', which is kept hot by magnetic field decay.