A new population of brown dwarfs     

M. R. S. Hawkins  C. Ducourant  H. R. A. Jones  & M. Rapaport 《Monthly notices of the Royal Astronomical Society》1998,294(3):505-512

In this paper we report the first results from a survey for low-mass stars and brown dwarfs, based on a photographic stack of around 100 Schmidt plates. This survey extends photographic searches by about 2 mag, and covers an area of 25 deg². Some 30 faint objects with large R − I colours were selected for further study, and were found to have very strong molecular absorption in their spectra, but only moderately red infrared colours. Five of these stars were selected for a parallax programme; three of these were found to be at a distance of around 45 pc, implying a very low luminosity. On the basis of their luminosity alone it is clear that these stars are field brown dwarfs, and we discuss their likely evolutionary status in the context of current models of low-mass stellar evolution.  相似文献   

Sodium beacon tip–tilt determination with Rayleigh-aided auxiliary telescope technique     

S. Esposito  A. Riccardi  & R. Ragazzoni 《Monthly notices of the Royal Astronomical Society》1998,294(3):489-493

The usefulness of tracking the Rayleigh portion of a mesospheric sodium laser guide star as reference for absolute tip–tilt recovery in the frame of the auxiliary telescopes technique is shown. This approach leads to the reduction of the ground occupation needed to attain a given sky coverage by more than one order of magnitude. Speed, tracking precision, and the number of auxiliary telescopes are also reduced, making this new approach a more attractive one. The use of a low-altitude Rayleigh spot reinforces the fundamental limitations affecting this and other techniques, thus degrading significantly the quality of the recovered tip–tilt. However, it is shown that, provided adequate care is taken in the collection and treatment of data, an interesting tilt signal can still be retrieved.  相似文献   

On the global structure of self-gravitating discs for softened gravity     

Jesper Sommer-Larsen  Henrik Vedel  & Uffe Hellsten 《Monthly notices of the Royal Astronomical Society》1998,294(3):485-488

The effects of gravitational softening on the global structure of self-gravitating discs in centrifugal equilibrium are examined in relation to hydrodynamical/gravitational simulations. The one-parameter spline softening proposed by Hernquist & Katz is used.
It is found that if the characteristic size of a disc, r , is comparable to or less than the gravitational softening length, ε, then the cross-section of the simulated disc is significantly larger than that of a no-softening (Newtonian) disc with the same mass and angular momentum.
We demonstrate, furthermore, that if r ≲ε/2 then the scaling relation r ∝ε^3/4 holds for a given mass and specific angular momentum distribution with mass. Finally, we compare some of the theoretical results obtained in this paper and a previous one with the results of numerical Tree-SPH simulations and find qualitative agreement.  相似文献   

Molecular hydrogen line emission from the reflection nebula Parsamyan 18     

Ryder  Allen  Burton  Ashley  & Storey 《Monthly notices of the Royal Astronomical Society》1998,294(2):338-346

The newly commissioned University of New South Wales Infrared Fabry–Perot (UNSWIRF) has been used to image molecular hydrogen emission at 2.12 and 2.25 μm in the reflection nebula Parsamyan 18. P 18 is known to exhibit low values of the (1–0)/(2–1) S(1) ratio, suggestive of UV-pumped fluorescence rather than thermal excitation by shocks. Our line ratio mapping reveals the full extent of this fluorescent emission from extended arc-like features, as well as a more concentrated thermal component in regions closer to the central exciting star. We show that the emission morphology, line fluxes and gas density are consistent with the predictions of photodissociation region (PDR) theory. Those regions with the highest intrinsic 1–0 S(1) intensities also tend to show the highest (1–0)/(2–1) S(1) line ratios. Furthermore, variations in the line ratio can be attributed to intrinsic fluctuations in the incident radiation field and/or the gas density, through the self-shielding action of H₂. An isolated knot of emission discovered just outside P 18, and having both an unusually high (1–0)/(2–1) S(1) ratio and relative velocity, provides additional evidence for an outflow source associated with P 18.  相似文献   

On the nature of the Galactic nebula We 1–12     

Kimeswenger 《Monthly notices of the Royal Astronomical Society》1998,294(2):312-314

In 1977, Weinberger published a list of 12 new extended possible planetary nebulae (PNe). Whereas, because of their characteristic morphology and/or the presence of a blue central star, almost all of them could easily be suspected to be genuine planetary nebulae, one object (No. 12) captivated because of its unusually bright central star. This find prompted Kaler & Feibelman to question the PN nature of this object (We 1–12) on the basis of IUE spectra. A definite conclusion could, however, not be drawn by them; thus, until now, the real nature of We 1–12 remained unsolved. For the first time, a spectral investigation of both the central star and the nebula is presented in this paper. It definitely shows that this intriguing object is an (isolated) H  ii region and its central star, as previously assumed, is an early B star which serves as the ionizing source. We 1–12, a part of which is coincident with a weak IRAS point source, is at a distance of 2–2.6 kpc and is reddened by E ( B − V )=0.6–0.8 mag.  相似文献   

The dwarf galaxy population of the Coma cluster to M_R=−11: a detailed description     

Neil Trentham 《Monthly notices of the Royal Astronomical Society》1998,293(1):71-88

We present the luminosity function and measurements of the scalelengths, colours and radial distribution of dwarf galaxies in the Coma cluster down to R =24. Our survey area is 674 arcmin²; this is the deepest and most detailed survey covering such a large area.
Our measurements agree with those of most previous authors at bright and intermediate magnitudes. The new results are as follows.
(1) Galaxies in the Coma cluster have a luminosity function φ( L )∝ L ^α that is steep (α∼−1.7) for −15< M_R <−11, and is shallower brighter than this. The curvature in the luminosity function at M_R ∼−15 is statistically significant.
(2) The galaxies that contribute most strongly to the luminosity function at −14< M_R <−12 have colours and scalelengths that are consistent with those of local dwarf spheroidal galaxies placed at the distance of Coma.
(3) These galaxies with −14< M_R <−12 have a colour distribution that is very strongly peaked at B − R =1.3. This is suggestive of a substantial degree of homogeneity in their star formation histories and metallicities.
(4) These galaxies with −14< M_R <−12 also appear to be more confined to the cluster core ( r ∼200 kpc) than the brighter galaxies. Alternatively, this observation may be explained in part or whole by the presence of an anomalously high number of background galaxies behind the cluster core. Velocity measurements of these galaxies would distinguish between these two possibilities.  相似文献   

The UVX quasar optical luminosity function and its evolution     

Pippa Goldschmidt  & Lance Miller 《Monthly notices of the Royal Astronomical Society》1998,293(1):107-112

The recently finished Edinburgh UVX quasar survey at B <18 is used together with other complete samples to estimate the shape and evolution of the optical luminosity function in the redshift range 0.3< z <2.2. There is a significantly higher space density of quasars at high luminosity and low redshift than previously found in the PG sample of Schmidt & Green, with the result that the shape of the luminosity function at low redshifts ( z <1) is seen to be consistent with a single power law. At higher redshifts the slope of the power law at high luminosities appears to steepen significantly. There does not appear to be any consistent break feature which could be used as a tracer of luminosity evolution in the population.  相似文献   

Heavy-element abundances in seven SC stars and several related stars     

Abia  & Wallerstein 《Monthly notices of the Royal Astronomical Society》1998,293(1):89-106

We employ spectra of resolution 20–35000 of seven SC stars, four S stars, two Ba stars and two K–M stars to derive abundances of a variety of elements from Sr to Eu relative to iron. Special attention is paid to Rb and Tc, and to the ratio of the heavy s-process species to the light s-process elements. Abundances are derived in LTE, both by using model atmospheres in which the carbon and oxygen abundances are nearly equal and by using curves of growth. Spectrum synthesis is used for critical lines such as the 5924-Å line of Tc and the 7800-Å line of Rb. For most of the heavy-element stars the enhancement of the s-process elements is about a factor of 10. The ratio of the heavy to light s-process species is not far from solar, except for RR Her for which the same ratio is +0.45 dex. For Tc the blending by other lines is severe. While we have probably detected the 5924-Å line, we can only present abundances in the less-than-or-equal-to category. For Rb, whose abundance is sensitive to the ⁸⁵Rb/⁸⁷Rb ratio and hence to the neutron density during s-process production, we find a considerable range of abundances, indicating a neutron density from 10⁶ to ≳10⁸ cm⁻³ for the SC stars. For the four S stars the range is from 10⁷ to ≳10⁸ cm⁻³. Recent calculations by Gallino et al. show that neutron densities near 10⁷ cm⁻³ favour the ¹³C source for neutrons, while densities greater than 10⁸ cm⁻³ may be associated with neutrons from the ²²Ne source.  相似文献   

The light curves of soft X-ray transients     

King  & Ritter 《Monthly notices of the Royal Astronomical Society》1998,293(1):L42-L48

We show that the light curves of soft X-ray transients (SXTs) follow naturally from the disc instability picture, adapted to take account of irradiation by the central X-ray source during the outburst. Irradiation prevents the disc from returning to the cool state until central accretion is greatly reduced. This happens only after most of the disc mass has been accreted by the central object, on a viscous time-scale, accounting naturally for the exponential decay of the outburst on a far longer time-scale (τ20–40 d) than seen in dwarf novae, without any need to manipulate the viscosity parameter α. The accretion of most of the disc mass in outburst explains the much longer recurrence time of SXTs compared with dwarf novae. This picture also suggests an explanation of the secondary maximum seen in SXT light curves about 50–75 d after the start of each outburst, since central irradiation triggers the thermal instability of the outer disc, adding to the central accretion rate one viscous time later. The X-ray outburst decay constant τ should on average increase with orbital period, but saturate at a roughly constant value ∼40 d for orbital periods longer than about a day. The bolometric light curve should show a linear rather than an exponential decay at late times (a few times τ). Outbursts of long-period systems should be entirely in the linear decay regime, as is observed in GRO J1744−28. UV and optical light curves should resemble the X-rays but have decay time-scales up to 2–4 times longer.  相似文献   

The biconical kiloparsec structure generated by nuclear starbursts     

Guillermo Tenorio-Tagle  & Casiana Muñoz-Tuñón 《Monthly notices of the Royal Astronomical Society》1998,293(3):299-305

Two-dimensional calculations of the hydrodynamics produced by nuclear starbursts, taking into consideration the accretion or infall of disc matter on to the heart of the starburst, are here shown to lead to stationary solutions that naturally account for the kpc-scale biconical X-ray and optically detected filamentary structure. The calculated flows are critically compared with former models and with observations of nuclear starbursts. For the infall models, we find that the mechanical energy power of the nuclear cluster must exceed a threshold value, imposed by the rate of disc mass accretion, to undergo blowout. This, combined with an initial mass function (IMF), is shown to regulate the minimum amount of mass that a starburst needs to generate kpc-scale outflows.  相似文献