Multisite campaign on the open cluster M67 – II. Evidence for solar-like oscillations in red giant stars

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.     

Gaussian process modelling of asteroseismic data

The measured properties of stellar oscillations can provide powerful constraints on the internal structure and composition of stars. To begin this process, oscillation frequencies must be extracted from the observational data, typically time series of the star's brightness or radial velocity. In this paper, a probabilistic model is introduced for inferring the frequencies and amplitudes of stellar oscillation modes from data, assuming that there is some periodic character to the oscillations, but that they may not be exactly sinusoidal. Effectively, we fit damped oscillations to the time series, and hence the mode lifetime is also recovered. While this approach is computationally demanding for large time series (>1500 points), it should at least allow improved analysis of observations of solar-like oscillations in subgiant and red giant stars, as well as sparse observations of semiregular stars, where the number of points in the time series is often low. The method is demonstrated on simulated data and then applied to radial velocity measurements of the red giant star  ξ Hydrae  , yielding a mode lifetime between 0.41 and 2.65 d with 95 per cent posterior probability. The large frequency separation between modes is ambiguous, however we argue that the most plausible value is 6.3 μHz, based on the radial velocity data and the star's position in the Hertzsprung–Russell diagram.     

The UV-Excess survey of the northern Galactic plane

The UV-Excess survey of the northern Galactic plane images a  10°× 185°  wide band, centred on the Galactic equator using the 2.5-m Isaac Newton Telescope in four bands (       i  5875) down to  ∼21–22  mag (     in       i  5875). The setup and data reduction procedures are described. Simulations of the colours of main-sequence stars, giant, supergiants, DA and DB white dwarfs and AM Canum Venaticorum stars are made, including the effects of reddening. A first look at the data of the survey (currently 30 per cent complete) is given.     

JHK standard stars for large telescopes: the UKIRT Fundamental and Extended lists

We present high-precision JHK photometry with the 3.8-m UK Infrared Telescope (UKIRT) of 82 standard stars, 28 from the widely used preliminary list known as the 'UKIRT Faint Standards', referred to here as the Fundamental List, and 54 additional stars referred to as the Extended List. The stars have 9.4< K <15.0 and all or most should be readily observable with imaging array detectors in normal operating modes on telescopes of up to 10-m aperture. Many are accessible from the southern hemisphere. Arcsec-accuracy positions (J2000, epoch ∼1998) are given, together with optical photometry and spectral types from the literature, where available, or inferred from the J − K colour. K -band finding charts are provided for stars with proper motions exceeding 0.3 arcsec yr⁻¹. We discuss some pitfalls in the construction of flat-fields for array imagers and a method to avoid them. On 30 nights between late 1994 and early 1998 the stars from the Fundamental List, which were used as standards for the whole programme, were observed on an average of 10 nights each, and those from the Extended List on an average of six nights. The average internal standard error of the mean results for the K magnitudes is 0.005 mag; for the J − H colours it is 0.003 mag for the Fundamental List stars and 0.005 mag for those of the Extended List; for H − K the average is 0.004 mag. The results are on the natural system of the IRCAM3 imager, which used a 256×256 InSb detector array with 'standard' JHK filters, behind gold-coated fore-optics and a gold- or silver-dielectric coated dichroic. We give colour transformations on to the CIT, Arcetri and LCO/Palomar NICMOS systems, and preliminary transformations on to the system defined by the new Mauna Kea Observatory near-infrared filter set.     

A frequency analysis of the p-mode pulsations of the Ap SrEu(Cr) star HD 119027

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.     

A Theoretical Probe for Excitation Mechanisms of Solar-like and Mira-like Oscillations of Stars

The linear stability analysis of the radial and non-radial oscillations for the evolutionary model of a star with the mass of 0.6∼3 M8 has been per- formed by using the nonlocal and time-dependent convection theory. The results show that the unstable low-temperature stars on the right side of the instabil- ity strip in the HR diagram can be divided into two groups. One is of the stars of solar-like oscillations, composed of the main-sequence dwarfs, subgiants, and the red giants with low- and intermediate-luminosity, which are unstable in the intermediate- and high-order (nr ≥ 12) p-modes, but stable in the low- order (nr ≤ 5) p-modes. Another is of the Mira-like stars, composed of the luminous red giants and AGB stars, which are just contrary to the solar-like stars, unstable in the low-order (nr ≤ 5) p-modes, but stable in the intermediate- and high-order (nr ≥ 12) p-modes. On the red edge of Cepheid (δ Scuti) insta- bility strip, the oscillations of solar-like and Mira-like stars can be explained uniformly by the coupling between convection and oscillations (CCO). For the low-temperature stars on the right side of the instability strip, the CCO is the dominant excitation and damping mechanism for the low- and intermediate-order p-modes, and the stochastic excitation of turbulence becomes important only for the high-order p-modes of solar-like oscillations.     

SDSS J160043.6+074802.9: a very rapid sdO pulsator

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.     

Two new pulsating hot subdwarf stars from the Edinburgh-Cape survey

We report the discovery of very rapid pulsations in two hot subdwarf stars from the Edinburgh-Cape blue object survey. The short periods, small amplitudes and multiperiodicity establish these stars as members of the class of rapidly-pulsating sdB stars. The spectrograms of both stars, however, show relatively strong He  ii 4686 and they are therefore more properly classified as sdOB. The light curve of EC 01541−1409 is dominated by two strong (∼1 per cent) variations with frequencies near 7114 and 7870 μHz (periods near 140.6 and 127.1 s), though at least five frequencies are present with amplitudes above about 0.002 mag. The light curve of EC 22221−3152 appears to be generated by at least 10 frequencies in the range 5670–11850 μHz (about 175–85 s) with amplitudes between about 0.01 and 0.001 mag, including the first overtone of the strongest variation. Somewhat surprisingly, this number of frequencies is detectable in observing runs as short as 3 h, probably due to the fact that the detected frequencies are well-separated.     

The stellar composition of the star formation region CMa R1 – I. Results from new photometric and spectroscopic classifications

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.     

Non-adiabatic oscillations of red giants

Using our non-local time-dependent theory of convection, the linear non-adiabatic oscillations of 10 evolutionary model series with masses of  1–3 M_⊙  are calculated. The results show that there is a red giant instability strip in the lower temperature side of the Hertzsprung–Russell diagram which goes along the sequences of the red giant branch and the asymptotic giant branch. For red giants of lower luminosities, pulsation instability is found at high order overtones; the lower order modes from the fundamental to the second overtone are stable. Towards higher luminosity and lower effective temperature, instability moves to lower order modes, and the amplitude growth rate of oscillations also grows. At the high luminosity end of the strip, the fundamental and the first overtone become unstable, while all the modes above the fourth order become stable. The excitation mechanisms have been studied in detail. It is found that turbulent pressure plays a key role for excitation of red variables. The frozen convection approximation is unavailable for the low temperature stars with extended convective envelopes. In any case, this approximation can explain neither the red edge of the Cepheid instability strip, nor the blue edge of the pulsating red giant instability strip. An analytic expression of a pulsation constant as a function of stellar mass, luminosity and effective temperature is presented from this work.     

Photometric variability of WC9 stars

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).     

M dwarfs at large heliocentric distances

We present an analysis of the faint M star population seen as foreground contaminants in deep extragalactic surveys. We use space-based data to separate such stars from high-redshift galaxies in a publicly available data set, and consider the photometric properties of the resulting sample in the optical and infrared. The inferred distances place these stars well beyond the scaleheight of the thick disc. We find strong similarities between this faint sample (reaching   i '_AB= 25  ) and the brighter disc M dwarf population studied by other authors. The optical–infrared properties of the bulk of our sources spanning 6000 Å-4.5 μm are consistent with those 5–10 mag brighter. We also present deep spectroscopy of faint M dwarf stars reaching continuum limits of i '_AB≈ 26, and measure absorption-line strengths in the CaH2 and TiO5 bands. Both photometrically and spectroscopically, our sources are consistent with metallicities as low as a tenth solar: metal-rich compared with halo stars at similar heliocentric distances. We comment on the possible massive astrophysical compact halo object (MACHO) identification of M stars at faint magnitudes.     

The infrared properties of barium stars

The infrared properties of barium stars are studied using published data in the K band and from IRAS . At 12 and 25 μm the emission from barium stars shows no excess over photospheric emission. Thus the claim made by Hakkila that some barium stars show evidence of the presence of warm (∼300  K ) circumstellar material is not supported. The 60-μm properties of barium stars are studied using survival analysis methods, and it is found that very few (3.7 ± 2.6 per cent) barium stars exhibit far-infrared excesses. Furthermore, it is found that the incidence of excess emission at 60 μm is lower in barium stars than for normal G and K giants. This may indicate that the mass-transfer event that is assumed to have taken place in barium stars has removed any cool circumstellar material that may have existed in these systems. Alternatively, it is suggested that the incidence of infrared excesses in normal G and K giants may have been over-estimated as a result of not fully accounting for foreground contamination by interstellar cirrus.     

Changes in the red giant and dusty environment of the recurrent nova RS Ophiuchi following the 2006 eruption

We present near-infrared spectroscopy of the recurrent nova RS Ophiuchi (RS Oph) obtained on several occasions after its latest outburst in 2006 February. The  1–5 μ  m spectra are dominated by the red giant, but the H  i , He  i and coronal lines present during the eruption are present in all our observations. From the fits of the computed infrared spectral energy distributions to the observed fluxes, we find   T _eff= 4200 ± 200   K for the red giant. The first overtone CO bands at  2.3 μ  m, formed in the atmosphere of the red giant, are variable. The spectra clearly exhibit an infrared excess due to dust emission longward of  5 μ  m; we estimate an effective temperature for the emitting dust shell of 500 K, and find that the dust emission is also variable, being beyond the limit of detection in 2007. Most likely, the secondary star in RS Oph is intrinsically variable.     

Do red giant stars pulsate in high overtones?

About 30 photometrically variable red giant stars have periods less than 10 d, as determined by the compilers of the Hipparcos Catalogue from Hipparcos photometric measurements. These periods, when combined with estimates of the radii and masses of these stars, and with pulsation theory, imply that these stars are pulsating in very high overtones. We present several pieces of evidence which suggest that the periods may be spurious, as a result of the particular aliasing properties of the Hipparcos photometry. We conclude that the evidence for high-overtone pulsation in red giant stars is equivocal.     

Asymptotic giant branch stars in the Phoenix dwarf galaxy

JHK _s near-infrared photometry of stars in the Phoenix dwarf galaxy is presented and discussed. Combining these data with the optical photometry of Massey et al. allows a rather clean separation of field stars from Phoenix members. The discovery of a Mira variable ( P = 425 d), which is almost certainly a carbon star, leads to an estimate of the distance modulus of 23.10 ± 0.18 that is consistent with other estimates and indicates the existence of a significant population of age ∼2 Gyr. The two carbon stars of Da Costa have   M _bol=−3.8  and are consistent with belonging to a population of similar age; some other possible members of such a population are identified. A Da Costa non-carbon star is  Δ K _s∼ 0.3  mag brighter than these two carbon stars. It may be an asymptotic giant branch star of the dominant old population. The nature of other stars lying close to it in the   K _s, ( J − K _s)  diagram needs studying.     

The red giant branch tip and bump of the Leo II dwarf spheroidal galaxy

We present V and I photometry of a  9.4 × 9.4 arcmin²  field centred on the dwarf spheroidal galaxy Leo II. The tip of the red giant branch (TRGB) is identified at   I ^TRGB= 17.83 ± 0.03  and adopting  〈[M/H]〉=−1.53 ± 0.2  from the comparison of RGB stars with Galactic templates, we obtain a distance modulus  ( m − M )₀= 21.84 ± 0.13  , corresponding to a distance   D = 233 ± 15 kpc  . Two significant bumps have been detected in the luminosity function of the RGB. The fainter bump (B1, at   V = 21.76 ± 0.05  ) is the RGB bump of the dominant stellar population while the actual nature of the brightest one (B2, at   V = 21.35 ± 0.05  ) cannot be firmly assessed on the basis of the available data; it may be due to the asymptotic giant branch clump of the main population or it may be a secondary RGB bump. The luminosity of the main RGB bump (B1) suggests that the majority of RGB stars in Leo II belong to a population that is ≳4 Gyr younger than the classical Galactic globular clusters. The stars belonging to the He-burning red clump are shown to be significantly more centrally concentrated than RR Lyrae and blue horizontal branch stars, probing the existence of an age/metallicity radial gradient in this remote dwarf spheroidal.     

High-frequency modes in solar-like stars

p-mode oscillations in solar-like stars are excited by the outer convection zone in these stars and reflected close to the surface. The p modes are trapped inside an acoustic cavity, but the modes only stay trapped up to a given frequency [known as the acoustic cut-off frequency  (ν_ac)  ] as modes with larger frequencies are generally not reflected at the surface. This means that modes with frequency larger than the acoustic cut-off frequency must be travelling waves. The high-frequency modes may provide information about the physics in the outer layers of the stars and the excitation source and are therefore highly interesting as it is the estimation of these two phenomena that cause some of the largest uncertainties when calculating stellar oscillations.
High-frequency modes have been detected in the Sun, in β Hydri and in α Cen A and α Cen B by smoothing the so-called echelle diagram and the large frequency separation as a function of frequency has been estimated. The large frequency separation has been compared with a simple model of the acoustic cavity which suggests that the reflectivity of the photosphere is larger at high frequency than predicted by standard models of the solar atmosphere and that the depth of the excitation source is larger than what has been estimated by other models and might depend on the order n and degree l of the modes.     

Period–luminosity relations for red supergiant variables – II. The distance to M101

We report the discovery of 42 red supergiant variables (RSVs) in the late-type spiral galaxy M101. Periods for the luminosity variation of these RSVs were determined from 20 epochs of ground-based CCD photometry in the Kron–Cousins R band obtained with the KPNO 2.1-m and WIYN 3.5-m telescopes over a span of three years. The periods found were in the range 200–1300 days. Using the relationship between the RSV periods and their luminosity in the Kron–Cousins I band, we estimate a reddening-corrected distance modulus to M101 of 29.40±0.16 mag (based on a distance modulus of 18.5±0.1 mag for the Large Magellanic Cloud). This distance is consistent with the Hubble Space Telescope Key Project Cepheid distances of 29.34±0.17 mag for the outer field of M101 and 29.21±0.17 mag for the inner field.