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1.
We report the discovery of very rapid pulsations in three sdB stars from the Edinburgh–Cape blue object survey. The short periods, small amplitudes and multi-periodicity clearly establish these stars as members of the EC 14026 class. EC 11583−2708 has pulsation periods near 149, 144 and 114 s, though evidence is presented that the 149-s period is resolved into two periods at 148.87 and 148.55 s by the full photoelectric data set. The amplitudes of the detected variations are in the range 0.002–0.006 mag. The light variation of EC 20338−1925 is dominated by a period near 147 s with a very large amplitude for a variable sdB star (0.025 mag), though four other frequencies are detected with periods near 168, 151, 141 and 135 s and amplitudes in the range 0.002–0.005 mag. The third star, EC 09582−1137, displays a light curve which is virtually a textbook example of frequency beating, being produced by two pulsations of almost equal amplitude (∼0.008 mag) and periods near 136.0 and 151.2 s.  相似文献   

2.
We present results from a two-week, single-site photometric campaign on the slowly pulsating sdB star, EC 21324−1346. Nine frequencies are detected between about 333 and 125 μHz (periods between 3000 and 8000 s) with amplitudes ranging from 0.003 to 0.001 mag; large for this class of star. Comparison with sparser, earlier data indicates that some of these oscillations are persistent, though at least one shows clear evidence of substantial amplitude variation.  相似文献   

3.
We report the discovery of pulsations in two DB stars found in the Edinburgh–Cape blue object survey. The light curve of EC 04207−4748 appears to be dominated by a strong variation at  2235 μHz  (447 s) and its first overtone near 4475 μHz (223 s). Two other peaks appear in the periodograms of all three data sets for this star; near  2370 μHz  (∼420 s) and  3000 μHz  (∼333 s), though these are less accurately defined. EC 05221−4725 is less easy to specify with the currently available data; it appears to have one coherent frequency near  1114 μHz  (898 s), but is obviously multiperiodic and probably has several more frequencies near the one clearly observed.  相似文献   

4.
We report the serendipitous discovery of the Sloan Digital Sky Survey (SDSS) star SDSS J160043.6+074802.9 to be a very rapid pulsator. The variation is dominated by a frequency near 8380 μHz (period = 119.33 s) with a large amplitude (0.04 mag) and its first harmonic at 16760 μHz (59.66 s; 0.005 mag). In between these frequencies, we find at least another eight variations with periods between 62 and 118 s and amplitudes between about 0.007 and 0.003 mag; weaker oscillations might also be present. Preliminary spectrograms from the performance verification phase of the Southern African Large Telescope indicate that SDSS J160043.6+074802.9 is a spectroscopic binary consisting of an sdO star and a late-type main-sequence companion. This makes it the first unambiguous detection of such an sdO star to pulsate, and certainly the first found to exhibit multifrequency variations.  相似文献   

5.
The sdB star PG 1336−018 is found to be a very short-period eclipsing binary system, remarkably similar to the previously unique system HW Vir. In addition, and unlike HW Vir, the sdB star in the PG 1336 system shows rapid oscillations of the type found in the recently discovered sdB pulsators, or EC 14026 stars. The orbital period, 0.101 0174 d, is one of the shortest known for a detached binary. Analysis of photoelectric and CCD photometry reveals pulsation periods near 184 and 141 s, with semi-amplitudes of ∼0.01 and ∼0.005 mag respectively. Both oscillations might have variable amplitude, and it is probable that other frequencies are present with amplitudes ∼0.003 mag or less. The 184- and 141-s pulsations are in the range of periods predicted by models for hot horizontal-branch stars. Analysis of medium-dispersion spectrograms yields T eff=33 000±1000 K and log g =5.7±0.1 for the sdB primary star, a radial velocity semi-amplitude K 1=78±3 km s−1 and a system velocity γ=6±2 km s−1. Spectrograms from the IUE Final Archive give T eff=33 000±3000 K and E ( B − V )=0.05 for log g =6.0 models. The derived angular radius leads to a distance of 710±50 pc for the system, and an absolute magnitude for the sdB star of +4.1±0.2. A preliminary analysis of U , V and R light curves indicates the orbital inclination to be near 81° and the relative radii to be r 1=0.19 and r 2=0.205. Assuming the mass of the sdB primary to be 0.5 M⊙ leads to a mass ratio q =0.3 for the system, and indicates that the secondary is a late-type dwarf of type ∼M5. As with HW Vir, it is necessary to invoke small limb-darkening coefficients and high albedos for the secondary star to obtain reasonable fits to the observed light curves.  相似文献   

6.
The pulsating DA white dwarfs are the coolest degenerate stars that undergo self-driven oscillations. Understanding their interior structure will help us to understand the previous evolution of the star. To this end, we report the analysis of more than 200 h of time-resolved CCD photometry of the pulsating DA white dwarf star EC 14012−1446 acquired during four observing epochs in three different years, including a coordinated three-site campaign. A total of 19 independent frequencies in the star's light variations together with 148 combination signals up to fifth order could be detected. We are unable to obtain the period spacing of the normal modes and therefore a mass estimate of the star, but we infer a fairly short rotation period of  0.61 ±0.03 d  , assuming the rotationally split modes are  ℓ= 1  . The pulsation modes of the star undergo amplitude and frequency variations, in the sense that modes with higher radial overtone show more pronounced variability and that amplitude changes are always accompanied by frequency variations. Most of the second-order combination frequencies detected have amplitudes that are a function of their parent mode amplitudes, but we found a few cases of possible resonantly excited modes. We point out the complications in the analysis and interpretation of data sets of pulsating white dwarfs that are affected by combination frequencies of the form   f A + f B − f C   intruding into the frequency range of the independent modes.  相似文献   

7.
Measuring solar-like oscillations in an ensemble of stars in a cluster, holds promise for testing stellar structure and evolution more stringently than just fitting parameters to single field stars. The most-ambitious attempt to pursue these prospects was by Gilliland et al. who targeted 11 turn-off stars in the open cluster M67 (NGC 2682), but the oscillation amplitudes were too small (<20 μmag) to obtain unambiguous detections. Like Gilliland et al. we also aim at detecting solar-like oscillations in M67, but we target red giant stars with expected amplitudes in the range 50–  500 μmag  and periods of 1 to 8 h. We analyse our recently published photometry measurements, obtained during a six-week multisite campaign using nine telescopes around the world. The observations are compared with simulations and with estimated properties of the stellar oscillations. Noise levels in the Fourier spectra as low as  27 μmag  are obtained for single sites, while the combined data reach  19 μmag  , making this the best photometric time series of an ensemble of red giant stars. These data enable us to make the first test of the scaling relations (used to estimate frequency and amplitude) with an homogeneous ensemble of stars. The detected excess power is consistent with the expected signal from stellar oscillations, both in terms of its frequency range and amplitude. However, our results are limited by apparent high levels of non-white noise, which cannot be clearly separated from the stellar signal.  相似文献   

8.
PB 8783 is a member of the newly discovered class of pulsating sdB stars, the EC 14026 stars. The potential of the pulsations of these stars as diagnostic probes of their interiors can only be realized fully by intensive campaigns of multisite time series photometry. To this end we have undertaken a campaign spanning 2 weeks and involving data from five different sites. 183 h of data were obtained. Frequency analysis yielded at least 11 pulsation periods in seven discrete groups between 94 and 136 s. Three of the periods in one specific group are equally spaced in frequency, with a spacing of 0.94 μHz. The frequency spacings amongst the members of other groups are similar to this. A rotation period of ∼12 d is implied. A general scheme for identifying the observed modes is suggested. The periods from a detailed model are in good overall agreement with the observed periods, but some difficulties remain. Three additional periods in the range 61–63 s, and with very low amplitude, were also found. No convincing interpretation of these could be devised.  相似文献   

9.
A new photometric and spectroscopic survey of the star formation region (SFR) CMa R1 is described. In a sample of 165 stars brighter than 13th mag, 88 stars were found to be probable members of the SFR. They are defined as early-type stars with E ( B − V )0.16 mag, which corresponds to a distance of about 1 kpc. 74 of the probable members are B stars. 19 stars are possibly associated with an IRAS point source. We derive a most probable distance of 1050±150 pc to the association. It appears that about 80 candidate members are pre-main-sequence stars with ages lower than 6 million years, while the main sequence extends over 6.0–7.6 mag, which is consistent with star formation starting about 8 million years ago and continuing until at least half a million years ago. Two bright B stars in the association (GU CMa and FZ CMa) seem to be much older and probably do not originate from the same star formation episode. The star formation efficiency appears to increase roughly monotonically with time up to half a million years ago. From our data, we conclude that only a minor fraction of the stars has been created through the scenario suggested by Herbst & Assousa, in which the members of CMa R1 form by compression of ambient material by a supernova shock wave. An extensive search for candidate members with H α emission did not reveal new Herbig Ae/Be candidates, so that the number of stars in this class seems to be limited to four: Z CMa, LkH α 218, LkH α 220 and possibly HD 53367.  相似文献   

10.
We present new high-speed, multisite photometric observations of the rapidly oscillating Ap star HD 119027 acquired over seven nights during 1996. A frequency analysis of these observations reveals the presence of oscillations at 1835, 1875, 1888, 1913, 1940, 1942 and (possibly) 1953 μHz. These frequencies are consistent with a spacing of either 13 or 26 μHz, depending on the reality of the oscillations at 1875 and 1953 μHz. The data in hand do not permit us to discriminate between the two possible spacings. If the smaller value of the spacing is correct, it suggests that HD 119027 is outside the main-sequence band. Two of the frequencies listed above are separated by only 1.95 μHz, suggesting that they are modes of ( n ,ℓ) and ( n  − 1, ℓ + 2), which in roAp stars is a quantity governed by the internal magnetic field.  相似文献   

11.
We present the results of extensive time-series photometry of the pulsating subdwarf B star KPD 2109 + 4401. Our data set consists of 29 data runs with a total length of 182.6 h over 31 d, collected at five observatories in 2004. These data comprised high signal-to-noise ratio observations acquired with larger telescopes and wider time-coverage observations obtained with smaller telescopes. They are sufficient to resolve the pulsation structure to 0.4 μHz and are the most extensive data set for this star to date. With these data, we identify eight pulsation frequencies extending from 4701 to 5481 μHz, corresponding to periods of 182–213 s. The pulsation frequencies and their amplitudes are examined over several time-scales with some frequencies showing amplitude variability.  相似文献   

12.
The light curve and spectra of Nova Centauri 1995 (V888 Cen) are analysed. The spectra were obtained a few days post-maximum. The nova is found to be a very fast nova, declining by 2 mag within about 5±2 d of maximum. The light curve shows strong oscillations in the transition region, of peak-to-peak amplitude about 1.5 mag and period 12–15 d. The light curve of Nova Centauri resembles closely that of Nova Aquilae 1918 (V603 Aql).
The early blue and red spectra obtained at Mt John show broad emission lines, many with P Cygni profiles. The absorption lines are found in two velocity systems at about −1765 and −3010 km s−1, in respectively the principal and diffuse-enhanced stages of spectral development, as defined by McLaughlin. Nova Centauri has many Fe  ii lines in emission, indicating that it is a member of the Williams Fe  ii class of classical novae.  相似文献   

13.
We report the discovery of large-amplitude (∼0.25 mag) pulsations in the bright ( V =12.8) sdB star, PG 1605+072. The dominant period is 480 s, but more than 20 periods were present on at least three separate occasions. Frequency analysis of the complete data set yields more than 30 periods. A few of these are harmonics or linear combinations of the strongest modes. Excluding the latter, the periods span a range of almost 400 s, which contrasts with the typical range <20 s for most other EC 14026 stars.
Analysis of multicolour photometry limited any cool companion to being a main-sequence star of type M0 or later. Balmer line profile fitting yielded an effective temperature of 32 100±1000 K and a log g of 5.25±0.10, significantly smaller than in the other stars of the EC 14026 class, and possibly indicative of a more evolved state. The lower gravity is probably responsible for the fact that the pulsation periods and amplitudes are respectively much longer and larger than in other stars of the class. This star is an obvious target for asteroseismological investigation using a multilongitude photometric campaign.  相似文献   

14.
Photometric observations over three seasons show HD 288313 to be a light variable with a 2.2636-d period. The observed V amplitudes lie in the range of 0.06–0.15 mag. The star showed appreciable changes in the brightness at maximum and minimum of the light curve from season to season. The (   b − y   ) colour did not show any significant variation during the photometric cycle. The light variation appears to be caused by the rotational modulation of stellar flux by cool starspots distributed asymmetrically across the stellar longitudes. The Hα line strength in HD 288313 varied drastically from completely filled-in emission to almost full absorption, that is typical of a normal star of similar spectral type. The Hα equivalent width is found to show a clear rotational modulation only occasionally. Most of the time, chromospheric active regions are distributed well across the stellar longitudes, thereby suppressing obvious rotational modulations. Broad-band linear polarization measurements show HD 288313 to be a short period, low-amplitude polarization variable. The polarization variation is, apparently, rotationally modulated. Dust grain scattering in a non-spherical circumstellar envelope of a star with inhomogeneities in the surface brightness distribution seems to be the mechanism operating in producing the observed polarization.  相似文献   

15.
CCD photometry in Johnson UBV and Strömgren uvby systems and medium-resolution spectroscopy of the galactic open cluster NGC 7128 are presented. Spectral types of the brightest 12 stars in the cluster field were determined based on equivalent widths of the H α and the He  i 6678-Å line. The spectroscopic observations also revealed two obvious and one probable Be-type stars showing H α emission. The analysis of the photometric diagrams gave a colour excess of E ( B − V )=1.03±0.06 mag, a distance modulus DM =13.0±0.2 mag and an age above 10 Myr. Time-resolved photometric observations obtained on one night resulted in the detection of short time-scale light variations of seven new and three already known variable stars in the cluster field.  相似文献   

16.
In this paper we report on further observations of the third and fourth kilohertz quasi-periodic oscillations (QPOs) in the power spectrum of the low-mass X-ray binary (LMXB) 4U 1636−53. These kilohertz QPOs are sidebands to the lower kilohertz QPO. The upper sideband has a frequency  55.5 ± 1.7 Hz  larger than that of the contemporaneously measured lower kilohertz QPO. Such a sideband has now been measured at a significance  >6σ  in the power spectra of three neutron-star LMXBs (4U 1636−53, 1728−34 and 1608−52). We also confirm the presence of a sideband at a frequency ∼55 Hz less than the frequency of the lower kilohertz QPO. The lower sideband is detected at a 3.5σ level only when the lower kilohertz QPO frequency is between 800 and 850 Hz. In that frequency interval, the sidebands are consistent with being symmetric around the lower kilohertz QPO frequency. The upper limit to the rms amplitude of the lower sideband is significantly lower than that of the upper sideband for lower kilohertz QPO frequencies >850 Hz. Symmetric sidebands are unique to 4U 1636−53. This might be explained by the fact that lower kilohertz QPO frequencies as high as 800–850 Hz are rare for 4U 1728−34 and 1608−52. Finally, we also measured a low-frequency QPO at a frequency of ∼43 Hz when the lower kilohertz QPO frequency is between 700 and 850 Hz. A similar low-frequency QPO is present in the power spectra of the other two systems for which a sideband has been observed. We briefly discuss the possibility that the sideband is caused by Lense–Thirring precession.  相似文献   

17.
The results of 37 h of high-speed photometry of KPD 2109+4401 are described. At least five periodicities in the range 182–198 s are consistently present in the observations, with amplitudes in the range 1–6 mmag. Results of simultaneous multicolour high-speed photometry with the Stiening photometer are also presented; these data could in principle be used for mode identification of the periodicities. The results of a preliminary study suggest that the pulsations may all be ℓ = 1 and 2 modes. Properties of the star are compared with those of the other EC 14026 stars.  相似文献   

18.
We report on two optical candidates for the counterpart to an X-ray source in the Small Magellanic Cloud , 1WGA J0053.8−7226, identified as a serendipitous X-ray source from the ROSAT Position Sensitive Proportional Counter (PSPC) archive, and also observed by the Einstein Imaging Proportional Counter . Its X-ray properties, namely the hard X-ray spectrum, flux variability and column density, indicate a hard, transient source, with a luminosity of ∼     XTE and ASCA observations have confirmed the source to be an X-ray pulsar, with a 46-s spin period. Our optical observations reveal two possible candidates within the error circle. Both exhibit strong H α and weaker H β emission. The optical colours indicate that both objects are Be-type stars. The Be nature of the stars implies that the counterpart is most likely a Be/X-ray binary system. Subsequent infrared (IR) photometry ( JHK ) of one of the objects shows that the source varies by at least 0.5 mag, while the     measured nearly simultaneously with the UBVRI and spectroscopic observations indicate an IR excess of ∼0.3 mag.  相似文献   

19.
Do some Wolf–Rayet stars owe their strong winds to something else besides radiation pressure? The answer to this question is still not entirely obvious, especially in certain Wolf–Rayet subclasses, mainly WN8 and WC9. Both of these types of Wolf–Rayet stars are thought to be highly variable, as suggested by observations, possibly due to pulsations. However, only the WN8 stars have so far been vigorously and systematically investigated for variability. We present here the results of a systematic survey during three consecutive weeks of 19 Galactic WC9 stars and one WC8 star for photometric variability in two optical bands, V and I . Of particular interest are the correlated variations in brightness and colour index in the context of carbon dust formation, which occurs frequently in WC9 and some WC8 stars. In the most variable case, WR76, we used this information to derive a typical dust grain size of  ∼ 0.1 μm  . However, most photometric variations occur at surprisingly low levels, and in fact almost half of our sample shows no significant variability at all above the instrumental level (  σ∼ 0.005– 0.01  mag).  相似文献   

20.
We report on the light variations of the infrared stars that were discovered recently in the Magellanic clusters NGC 419, 1783 and 1978. Their periods, of 528, 458 and 491 days, are among the longest known for carbon-rich Mira variables in the Clouds. All three IR stars were found to lie on the extension of the period– M bol relation derived from the shorter-period C-rich Miras while they were 0.45–0.70 mag fainter than the extension of the period– M K relation. Their main sequence masses were determined by isochrone fitting to be 1.5–1.6 M⊙, consistent with the prediction of the evolutionary models of Vassiliadis & Wood.  相似文献   

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