排序方式： 共有11条查询结果，搜索用时 15 毫秒
1.
M. Pozzo R. D. Jeffries T. Naylor E. J. Totten † S. Harmer‡ M. Kenyon§ 《Monthly notices of the Royal Astronomical Society》2000,313(2):L23-L27
We report the serendipitous discovery of a population of low-mass, pre-main-sequence (PMS) stars in the direction of the Wolf–Rayet/O-star binary system γ 2 Vel and the Vela OB2 association. We argue that γ 2 Vel and the low-mass stars are truly associated and approximately coeval, and that both are at distances between 360 and 490 pc, disagreeing at the 2 σ level with the recent Hipparcos parallax of γ 2 Vel, but consistent with older distance estimates. Our results clearly have implications for the physical parameters of the γ 2 Vel system, but also offer an exciting opportunity to investigate the influence of high-mass stars on the mass function and circumstellar disc lifetimes of their lower mass PMS siblings. 相似文献
2.
3.
A by-product of the APM high-redshift quasar survey was the discovery of several distant (20–100 kpc) N-type carbon stars at high galactic latitude. Following on from this, we have started a systematic all-sky survey at galactic latitudes ⊢ b ⊢>30° to find further examples of these rare objects, and we report here on the results from the first season of follow-up spectroscopy. Faint, high-latitude carbon (FHLC) giants make excellent probes of the kinematic structure of the outer Galactic halo. Therefore, in addition to detailed spectrophotometry covering a wide wavelength range, we have obtained high-resolution (∼1 Å) spectra centred on the CN bands at ∼8000 Å, and have derived accurate (≲10 km s−1 ) radial velocities for the known FHLC stars. From the initial phase of our survey covering ≈6500 deg2 , we find a surface density of faint N-type carbon stars in the halo of ≈1 per 200 deg2 , roughly a factor of 4 less than the surface density of CH-type carbon stars in the halo. Intermediate-age, N-type carbon stars seem unlikely to have formed in the halo in isolation from other star-forming regions, and one possibility that we are investigating is that they either arise from the disruption of tidally captured dwarf satellite galaxies or are a manifestation of the long-sought optical component of the Magellanic Stream. 相似文献
4.
5.
K. R. Briggs J. P. Pye R. D. Jeffries E. J. Totten 《Monthly notices of the Royal Astronomical Society》2000,319(3):826-836
Using deep exposures (∼105 s) with the ROSAT High Resolution Imager, we have performed flux-limited surveys for X-ray sources in the vicinity of the Hyades-age open stellar clusters NGC 6633 and IC 4756, detecting 31 and 13 sources, respectively. Our primary aim is to search for so-far unrecognized cluster members. We propose identifications or classifications (cluster member, field star, extragalactic field object) for the X-ray sources, based on published membership lists, and on X-ray:optical flux ratios and optical colour–magnitude diagrams. Results of simulating the expected X-ray-emitting source populations are compared with the ROSAT measurements and with the expected capabilities of the XMM mission. The simulations provide a novel method of comparing the activity levels of NGC 6633 and IC 4756 with that of the Hyades. The measurements and simulations confirm that cluster members are the major class of X-ray emitter in these fields at flux levels (0.1–2.4 keV), contributing ∼40 per cent of the total X-ray sources. We find six possible new members in NGC 6633 and four candidates in IC 4756; all require further observation to establish membership probability. 相似文献
6.
S. Harmer † R. D. Jeffries E. J. Totten J. P. Pye 《Monthly notices of the Royal Astronomical Society》2001,324(2):473-483
NGC 6633 is a young, open cluster with a similar age to the Hyades and Praesepe, but probably a lower metallicity. We present the results of ROSAT High Resolution Imager observations of an optically selected catalogue of likely members of NGC 6633. 8 out of 51 NGC 6633 members have been detected, with main-sequence spectral types A to G, above a threshold X-ray luminosity of ≈6–12×1028 erg s−1 . We find that NGC 6633 does not contain cool stars that are as X-ray luminous as the most active objects in the Hyades and that the median X-ray luminosity of F-G stars in NGC 6633 is less than that in the Hyades, but probably greater than in Praesepe. However, when X-ray activity is expressed as the X-ray to bolometric flux ratio we find that NGC 6633 and the Hyades are very similar and display similar peak levels of coronal activity. We attribute this discrepancy to a number of possible wide binary systems with higher X-ray (and bolometric) luminosities in the Hyades sample and either a low metallicity in NGC 6633, which makes its cool stars both X-ray and bolometrically less luminous at the same colour, or a distance to NGC 6633 that has been underestimated, which would decrease stellar X-ray luminosities without changing X-ray to bolometric flux ratios. 相似文献
7.
R. D. Jeffries E. J. Totten D. J. James 《Monthly notices of the Royal Astronomical Society》2000,316(4):950-960
We present high-resolution spectroscopy of a sample of 24 solar-type stars in the young (15–40 Myr), open cluster NGC 2547. We use our spectra to confirm cluster membership in 23 of these stars, to determine projected equatorial velocities and chromospheric activity, and to search for the presence of accretion discs. We find examples of both fast ( v e sin i >50 km s−1 ) and slow ( v e sin i <10 km s−1 ) rotators, but no evidence for active accretion in any of the sample. The distribution of projected rotation velocities is indistinguishable from the slightly older IC 2391 and IC 2602 clusters, implying similar initial angular momentum distributions and circumstellar disc lifetimes. The presence of very slow rotators indicates either that long (10–40 Myr) disc lifetimes or internal differential rotation are needed, or that NGC 2547 (and IC 2391/2602) were born with more slowly rotating stars than are presently seen in even younger clusters and associations. The solar-type stars in NGC 2547 follow a similar rotation–activity relationship to that seen in older clusters. X-ray activity increases until a saturation level is reached for v e sin i >15–20 km s−1 . We are unable to explain why this saturation level, of log( L x L bol )≃−3.3, is a factor of 2 lower than in other clusters, but rule out anomalously slow rotation rates or uncertainties in X-ray flux calculations. 相似文献
8.
9.
10.
E. J. Totten M. J. Irwin P. A. Whitelock 《Monthly notices of the Royal Astronomical Society》2000,314(3):630-640
We present proper-motion measurements for carbon stars found during the APM Survey for Cool Carbon Stars in the Galactic halo as reported in an earlier paper by Totten & Irwin. Measurements are obtained using a combination of POSSI, POSSII and UKST survey plates supplemented where necessary by CCD frames taken at the Isaac Newton Telescope. We find no significant proper motion for any of the new APM colour-selected carbon stars and so conclude that there are no dwarf carbon stars present within this sample. We also present proper-motion measurements for three previously known dwarf carbon stars and demonstrate that these measurements agree favourably with those previously quoted in the literature, verifying our method of determining proper motions. Results from a complimentary program of JHK photometry obtained at the South African Astronomical Observatory are also presented. Dwarf carbon stars are believed to have anomalous near-infrared colours, and this feature is used for further investigation of the nature of the APM carbon stars. Our results support the use of JHK photometry as a dwarf/giant discriminator and also reinforce the conclusion that none of the new APM-selected carbon stars is a dwarf. Finally, proper-motion measurements combined with extant JHK photometry are presented for a sample of previously known halo carbon stars, suggesting that one of these stars, CLS29, is likely to be a previously unrecognized dwarf carbon star. 相似文献