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1.
The proper motion of the faint (V = 25.6 mag) neutron star RXJ1856.5‐3754 as measured consistently with both the FORS1 optical imager on the ESO Very Large Telescope (VLT) as well as with the ROSAT High Resolution Imager is presented. With two VLT images obtained with a 0.95 year epoch difference in Apr 1999 and Apr 2000, a proper motion of μα = 0.326 ± 0.064″ per year to the east and no detectable motion in declination is found, consistent with simultaneous findings by Walter (2001) using three HST observations. Also, by comparing the positional differences between the strong X‐ray source RXJ1856.5‐3754 and other X‐ray sources detected in the same ROSAT High Resolution Imager field (observed in Oct 1994 and Oct 1997), a proper motion of 0.34 ± 0.12″ to the east is detected. This may be the first star, where a proper motion is clearly detected in X‐ray images. Additional VLT images taken in half‐year intervals give an upper limit to the parallax of ∼51 mas. Furthermore, in archived VLT observations, the first B‐band detection of this object is found with B = 25.14 ± 0.41 mag.  相似文献   

2.
We search for stellar and substellar companions of young nearby stars to investigate stellar multiplicity and formation of stellar and substellar companions. We detect common proper‐motion companions of stars via multi‐epoch imaging. Their companionship is finally confirmed with photometry and spectroscopy. Here we report the discovery of a new co‐moving (13 σ) stellar companion ∼17.8 arcsec (350AU in projected separation) north of the nearby star HD141272 (21 pc).With EMMI/NTT optical spectroscopy we determined the spectral type of the companion to be M3±0.5V. The derived spectral type as well as the near infrared photometry of the companion are both fully consistent with a M⊙ dwarf located at the distance of HD141272 (21 pc). Furthermore the photometry data rules out the pre‐main sequence status, since the system is consistent with the ZAMS of the Pleiades. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

3.
The semi‐regular variable star RU Vulpeculae (RU Vul) is being observed visually since 1935. Its pulsation period and amplitude are declining since ∼1954. A leading hypothesis to explain the period decrease in asymptotic giant branch (AGB) stars such as RU Vul is an ongoing flash of the He‐burning shell, also called a thermal pulse (TP), inside the star. In this paper, we present a CCD photometric light curve of RU Vul, derive its fundamental parameters, and test if the TP hypothesis can describe the observed period decline. We use CCD photometry to determine the present‐day pulsation period and amplitude in three photometric bands, and high‐resolution optical spectroscopy to derive the fundamental parameters. The period evolution of RU Vul is compared to predictions by evolutionary models of the AGB phase. We find that RU Vul is a metal‐poor star with a metallicity [M/H] = –1.59 ± 0.05 and an effective surface temperature of Teff = 3634 ± 20 K. The low metallicity of RU Vul and its kinematics indicate that it is an old, low‐mass member of the thick disc or the halo population. The present day pulsation period determined from our photometry is ∼108 d, the semiamplitude in the V ‐band is 0.39 ± 0.03 mag. The observed period decline is found to be well matched by an evolutionary AGB model with stellar parameters comparable to those of RU Vul. We conclude that the TP hypothesis is in good agreement with the observed period evolution of RU Vul. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

4.
We report new photometric observations of the ∼200 000 year old naked weak‐line run‐away T Tauri star Par 1724, located north of the Trapezium cluster in Orion. We observed in the broad band filters B, V, R, and I using the 90 cm Dutch telescope on La Silla, the 80 cm Wendelstein telescope, and a 25 cm telescope of the University Observatory Jena in Großschwabhausen near Jena. The photometric data in V and R are consistent with a ∼5.7 day rotation period due to spots, as observed before between 1960ies and 2000. Also, for the first time, we present evidence for a long‐term 9 or 17.5 year cycle in photometric data (V band) of such a young star, a cycle similar to that to of the Sun and other active stars (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

5.
We have performed deep, wide‐field imaging on a ∼0.4 deg2 field in the Pleiades (Melotte 22). The selected field was not yet target of a deep search for low mass stars and brown dwarfs. Our limiting magnitudes are R ∼ 22 mag and I ∼ 20 mag, sufficient to detect brown dwarf candidates down to 40 MJ. We found 197 objects, whose location in the (I, R – I) color magnitude diagram is consistent with the age and the distance of the Pleiades. Using CTK R and I as well as JHK photometry from our data and the 2MASS survey we were able to identify 7 new brown dwarf candidates. We present our data reduction technique, which enables us to resample, calibrate, and co‐add many images by just two steps. We estimate the interstellar extinction and the spectral type from our optical and the NIR data using a two‐dimensional χ2 fitting (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

6.
We present simultaneous UV , G , R and I monitoring of 19 M dwarfs that reveal a huge flare on the M9 dwarf with an amplitude in the UV of at least 6 mag. This is one of the strongest detections ever of an optical flare in an M star and one of the first in an ultracool dwarf (spectral types later than about M7). Four intermediate-strength flares  (Δ m UV < 4 mag)  were found in this and three other targets. For the whole sample we deduce a flare probability of 0.013 (rate of  0.022 h−1  ), and  0.049 (0.090 h−1)  for 2M1707+64 alone. Deviations of the flare emission from a blackbody is consistent with strong  Hα  line emission. We also confirm our previously found rotation period for 2M1707+64 and determine it more precisely to be  3.619 ± 0.015 h  .  相似文献   

7.
We present the first ever study of the bright star HD 1. The star was chosen arbitrarily just because of its outstanding Henry Draper number. Surprisingly, almost nothing is known about this bright 7.m4 star. Our observations were performed as part of the commissioning of the robotic telescope facility STELLA and its fiber‐fed high‐resolution optical echelle spectrograph SES in the years 2007–2010. We found long‐term radial velocity variations with a full amplitude of 9 km s–1 with an average velocity of –29.8 km s–1 and suggest the star to be a hitherto unknown single‐lined spectroscopic binary. A preliminary orbit with a period of 6.2 years (2279±69 days) and an eccentricity of 0.50±0.01 is given. Its rms uncertainty is just 73 m s–1. HD 1 appears to be a G9‐K0 giant of luminosity class IIIa with Teff = 4850±100 K, logg = 2.0±0.2, L ≈ 155 L⊙, a mass of 3.0±0.3 M⊙, a radius of 17.7 R⊙, and an age of ≈350 Myr. A relative abundance analysis led to a metallicity of [Fe/H] = –0.12 ± 0.09. The α ‐element silicon may indicate an overabundance of +0.13 though. The low strengths of some s‐process lines and a lower limit for the 12C/13C isotope ratio of ≥16 indicate that HD 1 is on the first ascend of the RGB. The absorption spectral lines appear rotationally broadened with a v sin i of 5.5±1.2 km s–1 but no chromospheric activity is evident. We also present photometric monitoring BV (RI)C data taken in parallel with STELLA. The star is likely a small‐amplitude (<10 mmag) photometric variable although no periodicity was found (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

8.
The knowledge of mass loss rates due to thermal winds in cool dwarfs is of crucial importance for modeling the evolution of physical parameters of main sequence single and binary stars. Very few, sometimes contradictory, measurements of such mass loss rates exist up to now. We present a new, independent method of measuring an amount of mass lost by a star during its past life. It is based on the comparison of the present mass distribution of solar type stars in an open cluster with the calculated distribution under an assumption that stars with masses lower than Mlim have lost an amount of mass equal to ΔM. The actual value of ΔM or its upper limit is found from the best fit. Analysis of four clusters: Pleiades, NGC 6996, Hyades and Praesepe gave upper limits for ΔM in three of them and the inconclusive result for Pleiades. The most restrictive limit was obtained for Praesepe indicating that the average mass loss rate of cool dwarfs in this cluster was lower than 6 × 10–11 M⊙/yr. With more accurate mass determinations of the solar type members of selected open clusters, including those of spectral type K, the method will provide more stringent limits for mass loss of cool dwarfs. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

9.
We present results of an optical and near‐infrared (IR) 1.8 deg2 survey in the Pleiades open cluster to search for substellar objects. From optical I ‐band images from the CFHT and J ‐band images from the 3.5m CAHA Telescope, we identify 18 faint and very red L brown dwarf candidates, with I > 20.9 and I – J > 3.2. The follow‐up observations of nine objects in the H ‐ and K s‐bands confirm that eight belong to the IR sequence of the cluster and the proper motion measurements of seven candidates confirm that they are Pleiades members. A preliminary estimation of the substellar mass spectrum dN/ dM in the form of a power law M c–α provides α = +0.57 ± 0.14. We extrapolate this function to estimate the number of planetary mass objects that could be present in the cluster down to 1 MJup. Sensitive searches combining far red and near‐IR observations may unveal these objects in a near future. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

10.
The results from a flare star investigation in the open cluster Alpha Persei are presented. Photographic flare star monitoring and CCD photometry of the discovered 4 new flare stars are made. The flare star activity phenomenon is restricted to the classical flare stars (UV Ceti type) as classified in the GCVS. The V/V‐I diagramme of the members of the cluster with the locations of the considered flare stars is given. Most of the flare stars are probable cluster members. The flare frequency determined from the Rozhen flare star monitoring is very low–one flare event occurs for 38.5 hours effective observing time. Comparison with the flare activity of the Pleiades is made because of the small difference in the age and distance of the clusters.  相似文献   

11.
We report the results of our search for magnetic fields in a sample of 16 field Be stars, the binary emission‐line B‐type star υ Sgr, and in a sample of fourteen members of the open young cluster NGC3766 in the Carina spiral arm. The sample of cluster members includes Be stars, normal B‐type stars and He‐strong/He‐weak stars. Nine Be stars have been studied with magnetic field time series obtained over ∼1 hour to get an insight into the temporal behaviour and the correlation of magnetic field properties with dynamical phenomena taking place in Be star atmospheres. The spectropolarimetric data were obtained at the European Southern Observatory with the multi‐mode instrument FORS1 installed at the 8m Kueyen telescope. We detect weak photospheric magnetic fields in four field Be stars, HD 62367, μ Cen, o Aqr, and ε Tuc. The strongest longitudinal magnetic field, 〈Bz〉 = 117 ± 38 G, was detected in the Be star HD 62367. Among the Be stars studied with time series, one Be star, λ Eri, displays cyclic variability of the magnetic field with a period of 21.12 min. The binary star υ Sgr, in the initial rapid phase of mass exchange between the two components with strong emission lines in the visible spectrum, is a magnetic variable star, probably on a timescale of a few months. The maximum longitudinal magnetic field 〈Bz〉 = –102 ± 10 G at MJD 54333.018 was measured using hydrogen lines. The cluster NGC3766 seems to be extremely interesting, where we find evidence for the presence of a magnetic field in seven early B‐type stars out of the observed fourteen cluster members (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

12.
Since the first optical detection of RX J0720.4–3125 various observations have been performed to determine astrometric and photometric data. We present the first detection of the isolated neutron star in the V Bessel filter to study the spectral energy distribution and derive a new astrometric position. At ESO Paranal we obtained very deep images with FORS 1 (three hours exposure time) of RX J0720.4–3125 in the V Bessel filter in January 2008. We derive the visual magnitude by standard star aperture photometry. Using sophisticated resampling software we correct the images for field distortions. Then we derive an updated position and proper motion value by comparing its position with FORS 1 observations of December 2000. We calculate a visual magnitude of V = 26.81 ± 0.09 mag, which is seven times in excess of what is expected from X‐ray data, but consistent with the extant U, B, and R data. Over about a seven year epoch difference we measured a proper motion of μ = 105.1 ± 7.4 mas yr–1 towards θ = 296.951° ± 0.0063° (NW), consistent with previous data (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

13.
We present B and V light curves of a large stellar flare obtained with the Wide Field Camera at the Isaac Newton 2.5‐m telescope (La Palma). The source object is a faint (mV = 21.38) foreground star in the field of the Andromeda galaxy, with its most probable spectral type being dM4. We provide an estimate of the total flare energy in the optical range and find it to be of the order of 1035 erg. The cooling phase of the large flare shows three additional weak flare‐like events, which we interpret as results of a triggering mechanism also observed on the Sun during large coronal mass ejections. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

14.
Because of the intense brightness of the OB‐type multiple star system σ Ori, the low‐mass stellar and substellar populations close to the centre of the very young σ Orionis cluster is poorly know. I present an IJHKs survey in the cluster centre, able to detect from the massive early‐type stars down to cluster members below the deuterium burning mass limit. The near‐infrared and optical data have been complemented with X‐ray imaging. Ten objects have been found for the first time to display high‐energy emission. Previously known stars with clear spectroscopic youth indicators and/or X‐ray emission define a clear sequence in the I vs. I – Ks diagram. I have found six new candidate cluster members that follow this sequence. One of them, in the magnitude interval of the brown dwarfs in the cluster, displays X‐ray emission and a very red J – Ks colour, indicative of a disc. Other three low‐mass stars have excesses in the Ks band as well. The frequency of X‐ray emitters in the area is 80±20 %. The spatial density of stars is very high, of up to 1.6±0.1 arcmin–2. There is no indication of lower abundance of substellar objects in the cluster centre. Finally, I also report two cluster stars with X‐ray emission located at only 8000–11000 AU to σ Ori AB, two sources with peculiar colours and an object with X‐ray emission and near‐infrared magnitudes similar to those of previously‐known substellar objects in the cluster. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

15.
We present the results of the optical photometric observations of the Pleiades (125 Myr) cluster. The aim of this research is to look for variability, both due to rotation of single stars and the presence of eclipsing binaries. We have used differential photometry and two different methods of statistical analysis of the data.We have found variability in the Pleiades cluster with amplitudes of about 50 mmag and uncertainties of 5 mmag until I=17.9, well within the brown dwarf regime. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

16.
The star ζ Ophiuchi is one of the brightest massive stars in the northern hemisphere and was intensively studied in various wavelength domains. The currently available observational material suggests that certain observed phenomena are related to the presence of a magnetic field. We acquired spectropolarimetric observations of ζ Oph with FORS 1 mounted on the 8‐m Kueyen telescope of the VLT to investigate if a magnetic field is indeed present in this star. Using all available absorption lines, we detect a mean longitudinal magnetic field 〈Bz〉all = 141 ± 45 G, confirming the magnetic nature of this star. We review the X‐ray properties of ζ Oph with the aim to understand whether the X‐ray emission of ζ Oph is dominated by magnetic or by wind instability processes (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

17.
The first CCD photometric investigation of the open cluster NGC 7296 up to now was performed within the narrow band Δa photometric system, which enables us to detect peculiar objects. A deeper investigation of that cluster followed, using the standard BV R ‐Bessel filter set. The age and E (B – V ) was determined independently to log t = 8.0 ± 0.1 and 0.15 ± 0.02, respectively by using Δa and broadband photometry. In total five Be/Ae objects and two metal‐weak stars showing significant negative Δa ‐values as well as one classical chemically peculiar star could be identified within that intermediate age open cluster. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

18.
We present an RI photometric survey covering an area of 430 arcmin2 around the multiple star σ Orionis. The observations were conducted with the 0.8 m IAC‐80 Telescope at the Teide Observatory. The survey limiting R and I magnitudes are 22.5 and 21, and completeness magnitudes 21 and 20, respectively. We have selected 53 candidates from the I vs. R–I colour‐magnitude diagram (I = 14–20) that follow the previously known photometric sequence of the cluster. Adopting an age of 2–4 Myr for the cluster, we find that these objects span a mass range from 0.35 M⊙ to 0.015 M⊙. We have performed J‐band photometry of 52 candidates and Ks photometry for 12 of them, with the result that 50 follow the expected infrared sequence for the cluster, thus confirming with great confidence that the majority of the candidates are bona fide members. JHKs photometry from the Two Micron All Sky Survey (2MASS) is available for 50 of the candidates and are in good agreement with our data. Out of 48 candidates, which have photometric accuracies better than 0.1 mag in all bands, only three appear to show near‐infrared excesses. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

19.
The Hipparcos Space Astrometry Mission photometric observations of V398 Lac, led to the discovery of its variability, allowing to classify it as an eclipsing binary with an orbital period of about 5.4 days. This prompted us to acquire highresolution échelle spectra with the aim of performing accurate radial velocity measurements and to determine the main physical parameters of the system's components. We present, for the first time, a double‐lined radial velocity curve and determine the orbital and physical parameters of the two components, that can be classified both as late B‐type stars. In particular, we obtained an orbital inclination i ∼ 85°. With this value of the inclination, we deduced masses M1 = 3.83±0.35 M⊙ andM2 = 3.29±0.32 M⊙, and radii R1 = 4.89±0.18 R⊙ and R2 = 2.45±0.11 R⊙ for the more massive and less massive components, respectively. Both components are well inside their own Roche lobes. The mass ratio is M2/M1 ∼ 0.86. We derived also the projected rotational velocities as v1 sin i = 79±2 km s–1 and v2 sin i = 19±2 km s–1. Our measurements indicate that the rotation of the primary star is essentially pseudo‐synchronized with the orbital velocity at the periastron, while the secondary appears to rotate very slowly and has not yet attained synchronization. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

20.
Light curve with the flare observed in the Pleiades cluster with the Schmidt‐Teleskop‐Kamera at the University Observatory Jena (see M. Moualla et al., this issue, p. 661).  相似文献   

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