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1.
We present the results of a radial velocity (RV) survey of 14 brown dwarfs (BDs) and very low-mass (VLM) stars in the Upper Scorpius OB association (UScoOB) and three BD candidates in the ρ Ophiuchi dark cloud core. We obtained high-resolution echelle spectra at the Very Large Telescope using the Ultraviolet and Visual Echelle Spectrograph (UVES) at two different epochs for each object, and measured the shifts in their RVs to identify candidates for binary/multiple systems in the sample. The average time separation of the RV measurements is 21.6 d, and our survey is sensitive to the binaries with separation <0.1 au. We found that four out of 17 objects (or 24+16−13 per cent by fraction) show a significant RV change in 4–33 d time-scale, and are considered as binary/multiple 'candidates'. We found no double-lined spectroscopic binaries in our sample, based on the shape of cross-correlation curves. The RV dispersion of the objects in UScoOB is found to be very similar to that of the BD and VLM stars in Chamaeleon I (Cha I). We also found the distribution of the mean rotational velocities ( v   sin   i ) of the UScoOB objects is similar to that of the Cha I, but the dispersion of v   sin   i is much larger than that of the Cha I objects.  相似文献   

2.
We present the results of a spectroscopic multisite campaign for the β Cephei star 12 (DD) Lacertae. Our study is based on more than thousand high-resolution high S/N spectra gathered with eight different telescopes in a time span of 11 months. In addition, we make use of numerous archival spectroscopic measurements. We confirm 10 independent frequencies recently discovered from photometry, as well as harmonics and combination frequencies. In particular, the slowly pulsating B-stars (SPB)-like g -mode with frequency 0.3428 d−1 reported before is detected in our spectroscopy. We identify the four main modes as  (ℓ1, m 1) = (1, 1), (ℓ2, m 2) = (0, 0), (ℓ3, m 3) = (1, 0)  and  (ℓ4, m 4) = (2, 1)  for   f 1= 5.178 964 d−1, f 2= 5.334 224 d−1, f 3= 5.066 316 d−1  and   f 4= 5.490 133 d−1  , respectively. Our seismic modelling shows that f 2 is likely the radial first overtone and that the core overshooting parameter  αov  is lower than 0.4 local pressure scale heights.  相似文献   

3.
We present the results of a deep optical–near-infrared (optical–NIR) multi-epoch survey covering 2.5 deg2 of the Pleiades open star cluster to search for new very low-mass brown dwarf members. A significant (∼5 yr) epoch difference exists between the optical (CFH12k I -, Z -band) and NIR (UKIRT WFCAM J -band) observations. We construct I , I − Z and Z , Z − J colour–magnitude diagrams to select candidate cluster members. Proper motions are computed for all candidate members and compared to the background field objects to further refine the sample. We recover all known cluster members within the area of our survey. In addition, we have discovered nine new candidate brown dwarf cluster members. The seven faintest candidates have red Z − J colours and show blue NIR colours. These are consistent with being L- and T-type Pleiads. Theoretical models predict their masses to be around 11 M Jup.  相似文献   

4.
We present a measurement of the cluster X-ray luminosity–temperature ( L – T ) relation out to high redshift ( z ∼0.8). Combined ROSAT PSPC spectra of 91 galaxy clusters detected in the Wide Angle ROSAT Pointed Survey (WARPS) are simultaneously fitted in redshift and luminosity bins. The resulting temperature and luminosity measurements of these bins, which occupy a region of the high-redshift L – T relation not previously sampled, are compared with existing measurements at low redshift in order to constrain the evolution of the L – T relation. We find the best fit to low-redshift ( z <0.2) cluster data, at T >1 keV, to be L ∝ T 3.15±0.06. Our data are consistent with no evolution in the normalization of the L – T relation up to z ∼0.8. Combining our results with ASCA measurements taken from the literature, we find η =0.19±0.38 (for Ω0=1, with 1 σ errors) where L Bol∝(1+ z ) η T 3.15, or η =0.60±0.38 for Ω0=0.3. This lack of evolution is considered in terms of the entropy-driven evolution of clusters. Further implications for cosmological constraints are also discussed.  相似文献   

5.
Continuum-subtracted dereddened images in the light of several atomic lines show the presence of an extended bipolar nebula surrounding η Carinae with size ∼100×45 arcsec2 (1.3×0.5 pc2). This feature is best delineated in [O  iii ] 5007. The geometrical disposition and mass of the shell suggest that it was formed by mass ejections from η Carinae. The dynamic age of the nebula is ∼13 000/ V 7 yr, where V 7 is the mean expansion velocity in 100 km s−1, and its mass is between 5 and 10 M⊙. The nebula is photoionized and composed of unprocessed material. The major axes of the nebula and of the Homunculus are nearly perpendicular. We also report the discovery of elongated emission knots prominent in [N  ii ] located 64 to 100 arcsec away from η Carinae, which implies that they were ejected either centuries ago or at a more recent date but with extremely large velocities.  相似文献   

6.
NGC 6611 is the massive young cluster (2–3 Myr) that ionizes the Eagle Nebula. We present very deep photometric observations of the central region of NGC 6611 obtained with the Hubble Space Telescope and the following filters: ACS/WFC F775W and F850LP and NIC2 F110W and F160W, loosely equivalent to ground-based IZJH filters. This survey reaches down to   I ∼ 26 mag  . We construct the initial mass function (IMF) from  ∼1.5 M⊙  well into the brown dwarf regime (down to  ∼0.02 M⊙  ). We have detected 30–35 brown dwarf candidates in this sample. The low-mass IMF is combined with a higher-mass IMF constructed from the ground-based catalogue from Oliveira et al. We compare the final IMF with those of well-studied star-forming regions: we find that the IMF of NGC 6611 more closely resembles that of the low-mass star-forming region in Taurus than that of the more massive Orion Nebula Cluster. We conclude that there seems to be no severe environmental effect in the IMF due to the proximity of the massive stars in NGC 6611.  相似文献   

7.
We present a detailed calculation of the evolution of low-mass (<0.25 M⊙) helium white dwarfs. These white dwarfs (the optical companions to binary millisecond pulsars) are formed via long-term, low-mass binary evolution. After detachment from the Roche lobe, the hot helium cores have a rather thick hydrogen layer with mass between 0.01 and 0.06 M⊙. As a result of mixing between the core and outer envelope, the surface hydrogen content ( X surf) is 0.5–0.35 , depending on the initial value of the heavy element Z and the initial secondary mass. We found that the majority of our computed models experience one or two hydrogen shell flashes. We found that the mass of the helium dwarf in which the hydrogen shell flash occurs depends on the chemical composition. The minimum helium white dwarf mass in which a hydrogen flash takes place is 0.213 M⊙ ( Z =0.003), 0.198 M⊙ ( Z =0.01), 0.192 M⊙ ( Z =0.02) or 0.183 M⊙ ( Z =0.03). The duration of the flashes (independent of chemical composition) is between a few ×106 and a few ×107 yr. In several flashes the white dwarf radius will increase so much that it forces the model to fill its Roche lobe again. Our calculations show that the cooling history of the helium white dwarf depends dramatically on the thickness of the hydrogen layer. We show that the transition from a cooling white dwarf with a temporarily stable hydrogen-burning shell to a cooling white dwarf in which almost all residual hydrogen is lost in a few thermal flashes (via Roche lobe overflow) occurs between 0.183 and 0.213 M⊙ (depending on the heavy element value).  相似文献   

8.
We present CCD BVI photometry for the southern open cluster NGC 2489 and its surrounding field. The sample consists of 2182 stars measured in an area of 13.6 × 13.6 arcmin2, extending down to   V ∼ 21.5  . These data are supplemented with CORAVEL radial-velocity observations for seven red giant candidates. A cluster angular radius of 6.7 ± 0.6 arcmin, equivalent to 3.5 ± 0.3 pc, is estimated from star counts carried out inside and outside the cluster region. The comparison of the cluster colour–magnitude diagrams with isochrones of the Padova group yields   E ( B − V ) = 0.30 ± 0.05, E ( V − I ) = 0.40 ± 0.05  and   V − M V = 12.20 ± 0.25  for log   t = 8.70 ( t = 500+130−100 Myr)  and   Z = 0.019  . NGC 2489 is then located at 1.8 ± 0.3 kpc from the Sun and 25 pc below the Galactic plane. The analysis of the kinematical data allowed us to confirm cluster membership for six red giants, one of them being a spectroscopic binary. A mean radial velocity of 38.13 ± 0.33 km s−1 was derived for the cluster red giants. The properties of a sample of open clusters aligned along the line of sight of NGC 2489 are examined.  相似文献   

9.
I have re-visited the spatial distribution of stars and high-mass brown dwarfs in the σ Orionis (σ Ori) cluster (∼3 Ma, ∼360 pc). The input was a catalogue of 340 cluster members and candidates at separations less than 30 arcmin to σ Ori AB. Of them, 70 per cent have features of extreme youth. I fitted the normalized cumulative number of objects counting from the cluster centre to several power-law, exponential and King radial distributions. The cluster seems to have two components: a dense core that extends from the centre to   r ≈  20 arcmin and a rarified halo at larger separations. The radial distribution in the core follows a power law proportional to r 1, which corresponds to a volume density proportional to   r −2  . This is consistent with the collapse of an isothermal spherical molecular cloud. The stars more massive than  3.7 M⊙  concentrate, however, towards the cluster centre, where there is also an apparent deficit of very low mass objects  ( M < 0.16 M⊙)  . Last, I demonstrated through Monte Carlo simulations that the cluster is azimuthally asymmetric, with a filamentary overdensity of objects that runs from the cluster centre to the Horsehead Nebula.  相似文献   

10.
The amount of mass contained in low-mass objects is investigated anew. Instead of using a mass–luminosity relation to convert a luminosity function to a mass function, I predict the mass–luminosity relation from assumed mass functions and the luminosity functions of Jahreiss & Wielen and Gould, Bahcall & Flynn. Comparison of the resulting mass–luminosity relations with data for binary stars constrains the permissible mass functions. If the mass function is assumed to be a power law, the best-fitting slope lies either side of the critical slope, α =−2, below which the mass in low-mass objects is divergent, depending on the luminosity function adopted. If these power-law mass functions are truncated at 0.001 M⊙, the contribution to the local density from stars lies between 0.013 and 0.10 M⊙ pc−3 depending on the mass at which the mass function is normalized and the adopted value of α . Recent dynamical estimates of the local mass density rule out stellar mass densities above ∼0.05 M⊙ pc−3. Hence, power laws steeper than α =−2 that extend down to 0.001 M⊙ are allowed only if one adopts an implausible normalization of the mass function. If the mass function is generalized from a power law to a low-order polynomial in log( M ), the mass in stars with M <0.1 M⊙ is either negligible or strongly divergent, depending on the order of the polynomial adopted.  相似文献   

11.
An inspection of a GHRS/ HST spectrum of the symbiotic star RR Telescopii reveals the presence of the [Al  ii ] 3s21S – 3s3p 3P2 line at a vacuum wavelength of 2661.06±0.08 Å, 8.89±0.08 Å away from the Al  ii ] 3s21S – 3s3p 3P1 intercombination transition at 2669.95 Å, in good agreement with the theoretical prediction of Δ λ =8.80 Å. We also find that the Al  ii ] line profile is asymmetric, showing a strong low-density component with a weak high-density wing, redshifted by 30 km s−1, in agreement with the findings of Schild & Schmid, which were based on optical observations. Our measurement of the emission-line ratio R I (2661.06 Å)/ I (2669.95 Å)=0.027±0.003 implies log  N e=5.8±0.2, in good agreement with the densities found from other ions, such as Si  iii . These results provide strong evidence that we have detected the [Al  ii ] line, the first time (to our knowledge) that this feature has been reliably identified in an astrophysical or laboratory spectrum.  相似文献   

12.
We present a simple physical mechanism that can account for the observed stellar mass spectrum for masses M ∗≳0.5 M⊙ . The model depends solely on the competitive accretion that occurs in stellar clusters where each star's accretion rate depends on the local gas density and the square of the accretion radius. In a stellar cluster, there are two different regimes depending on whether the gas or the stars dominate the gravitational potential. When the cluster is dominated by cold gas, the accretion radius is given by a tidal-lobe radius. This occurs as the cluster collapses towards a ρ  ∝  R −2 distribution. Accretion in this regime results in a mass spectrum with an asymptotic limit of γ =−3/2 (where Salpeter is γ =−2.35) . Once the stars dominate the potential and are virialized, which occurs first in the cluster core, the accretion radius is the Bondi–Hoyle radius. The resultant mass spectrum has an asymptotic limit of γ =−2 with slightly steeper slopes ( γ ≈−2.5) if the stars are already mass-segregated. Simulations of accretion on to clusters containing 1000 stars show that, as expected, the low-mass stars accumulate the majority of their masses during the gas-dominated phase whereas the high-mass stars accumulate the majority of their masses during the stellar-dominated phase. This results in a mass spectrum with a relatively shallow γ ≈3/2 power law for low-mass stars and a steeper power law for high-mass stars −2.5≲ γ ≤−2 . This competitive accretion model also results in a mass-segregated cluster.  相似文献   

13.
We present V and I photometry of two open clusters in the LMC down to V ∼26. The clusters were imaged with the Wide Field and Planetary Camera 2 (WFPC2) on board the Hubble Space Telescope ( HST ), as part of the Medium Deep Survey Key Project. Both are low-luminosity ( MV ∼−3.5), low-mass ( M ∼103 M⊙) systems. The chance discovery of these two clusters in two parallel WFPC2 fields suggests a significant incompleteness in the LMC cluster census near the bar. One of the clusters is roughly elliptical and compact, with a steep light profile, a central surface brightness μ V (0)∼20.2 mag arcsec−2, a half-light radius r hl∼0.9 pc (total visual major diameter D ∼3 pc) and an estimated mass M ∼1500 M⊙. From the colour–magnitude diagram and isochrone fits we estimate its age as τ∼(2–5)×108 yr. Its mass function has a fitted slope of Γ=Δlogφ( M )/Δlog M =−1.8±0.7 in the range probed (0.9≲ M /M⊙≲4.5). The other cluster is more irregular and sparse, having shallower density and surface brightness profiles. We obtain Γ=−1.2±0.4, and estimate its mass as M ∼400 M⊙. A derived upper limit for its age is τ≲5×108 yr. Both clusters have mass functions with slopes similar to that of R136, a massive LMC cluster, for which HST results indicate Γ∼−1.2. They also seem to be relaxed in their cores and well contained in their tidal radii.  相似文献   

14.
In this paper we report on optical spectroscopic observations of the low-mass X-ray binary 2S 0921–630 obtained with the Very Large Telescope. We found sinusoidal radial velocity variations of the companion star with a semi-amplitude of  99.1 ± 3.1 km s−1  modulated on a period of 9.006 ± 0.007 d, consistent with the orbital period found previously for this source, and a systemic velocity of  44.4 ± 2.4 km s−1  . Owing to X-ray irradiation, the centre of light measured by the absorption lines from the companion star is probably shifted with respect to the centre of mass. We try to correct for this using the so-called K -correction. Conservatively applying the maximum correction possible and using the previously measured rotational velocity of the companion star, we find a lower limit to the mass of the compact object in 2S 0921–630 of   MX sin3 i > 1.90 ± 0.25 M⊙  (1σ errors). The inclination in this system is well constrained since partial eclipses have been observed in X-ray and optical bands. For inclinations in the range  60° < i < 90°  we find  1.90 ± 0.25 < MX < 2.9 ± 0.4 M⊙  . However, using this maximum K -correction we find that the ratio between the mass of the companion star and that of the compact object, q , is 1.32 ± 0.37, implying super-Eddington mass-transfer rates; however, evidence for that has not been found in 2S 0921–630. We conclude that the compact object in 2S 0921–630 is either a (massive) neutron star or a low-mass black hole.  相似文献   

15.
In an attempt to model the accretion on to a neutron star in low-mass X-ray binaries, we present 2D hydrodynamical models of the gas flow in close vicinity of the stellar surface. First, we consider a gas pressure-dominated case, assuming that the star is non-rotating. For the stellar mass we take   M star= 1.4 × 10−2 M⊙  and for the gas temperature   T = 5 × 106 K  . Our results are qualitatively different in the case of a realistic neutron star mass and a realistic gas temperature of T ≃ 108 K, when the radiation pressure dominates. We show that to get the stationary solution in a latter case, the star most probably has to rotate with the considerable velocity.  相似文献   

16.
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   

17.
We report on Two-Micron All-Sky Survey (2MASS) J01542930+0053266, a faint eclipsing system composed of two M dwarfs. The variability of this system was originally discovered during a pilot study of the 2MASS Calibration Point Source Working Data base. Additional photometry from the Sloan Digital Sky Survey yields an eight-passband light curve from which we derive an orbital period of  2.639 0157 ± 0.000 0016  d. Spectroscopic followup confirms our photometric classification of the system, which is likely composed of M0 and M1 dwarfs. Radial velocity measurements allow us to derive the masses  (M1= 0.66 ± 0.03 M⊙; M2= 0.62 ± 0.03 M⊙)  and radii  (R1= 0.64 ± 0.08 R⊙; R2= 0.61 ± 0.09 R⊙)  of the components, which are consistent with empirical mass–radius relationships for low-mass stars in binary systems. We perform Monte Carlo simulations of the light curves which allow us to uncover complicated degeneracies between the system parameters. Both stars show evidence of Hα emission, something not common in early-type M dwarfs. This suggests that binarity may influence the magnetic activity properties of low-mass stars; activity in the binary may persist long after the dynamos in their isolated counterparts have decayed, yielding a new potential foreground of flaring activity for next generation variability surveys.  相似文献   

18.
Intermediate resolution spectroscopy from the European Southern Observatory Very Large Telescope is analysed for 63 photometrically selected low-mass  (0.08–0.30 M⊙)  candidates of the open cluster NGC 2547. We have confirmed membership for most of these stars using radial velocities, and found that lithium remains undepleted for cluster stars with   I > 17.54 ± 0.14  and   Ks > 14.86 ± 0.12  . From these results, several pre–main-sequence evolutionary models give almost model independent ages of 34–36 Myr, with a precision of 10 per cent. These ages are only slightly larger than the ages of 25–35(±5) Myr obtained using the same models to fit isochrones to higher mass stars descending towards the zero-age main-sequence, both in empirically calibrated and theoretical colour–magnitude diagrams. This agreement between age determinations in different mass ranges is an excellent test of the current generation of low-mass pre–main-sequence stellar models and lends confidence to ages determined with either method between 30 and 120 Myr.  相似文献   

19.
We compute the continuous part of the ideal-magnetohydrodynamic (ideal-MHD) frequency spectrum of a polar mountain produced by magnetic burial on an accreting neutron star. Applying the formalism developed by Hellsten & Spies, extended to include gravity, we solve the singular eigenvalue problem subject to line-tying boundary conditions. This spectrum divides into an Alfvén part and a cusp part. The eigenfunctions are chirped and anharmonic with an exponential envelope, and the eigenfrequencies cover the whole spectrum above a minimum ωlow. For equilibria with accreted mass  1.2 × 10−6≲ M a/M⊙≲ 1.7 × 10−4  and surface magnetic fields  1011≲ B */G ≲ 1013, ωlow  is approximately independent of   B *  , and increases with M a. The results are consistent with the Alfvén spectrum excited in numerical simulations with the zeus-mp solver. The spectrum is modified substantially by the Coriolis force in neutron stars spinning faster than ∼100 Hz. The implications for gravitational-wave searches for low-mass X-ray binaries are considered briefly.  相似文献   

20.
We present our findings based on a detailed analysis of the binaries of the Hyades, in which the masses of the components are well known. We fit the models of the components of a binary system to observations so as to give the observed total V and B − V of that system and the observed slope of the main sequence in the corresponding parts. According to our findings, there is a very definite relationship between the mixing-length parameter and the stellar mass. The fitting formula for this relationship can be given as  α= 9.19( M /M⊙− 0.74)0.053− 6.65  , which is valid for stellar masses greater than  0.77 M⊙  . While no strict information is gathered for the chemical composition of the cluster, as a result of degeneracy in the colour–magnitude diagram, by adopting   Z = 0.033  and using models for the components of 70 Tau and θ2 Tau we find the hydrogen abundance to be   X = 0.676  and the age to be 670 Myr. If we assume that   Z = 0.024  , then   X = 0.718  and the age is 720 Myr. Our findings concerning the mixing-length parameter are valid for both sets of the solution. For both components of the active binary system V818 Tau, the differences between radii of the models with   Z = 0.024  and the observed radii are only about 4 per cent. More generally, the effective temperatures of the models of low-mass stars in the binary systems studied are in good agreement with those determined by spectroscopic methods.  相似文献   

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