Publications (55)61.96 Total impact
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Article: The discovery of two new rapidly oscillating Ap stars, HD 92499 and HD 143487★
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ABSTRACT: We report the discovery of short periodic radial velocity variations in the stars HD 92499 and HD 143487. Both stars show strong magnetic fields and large overabundances of rare earth elements and belong to the class of cool chemically peculiar stars of the main sequence. They are therefore new rapidly oscillating Ap stars. Pulsations were detected from analysis of high time resolution spectra obtained with the European Southern Observatory Very Large Telescope by using a cross-correlation method for large spectral bands and from combinations of lines belonging to rare earth elements. The amplitudes of the pulsations are small and do not exceed several dozens of m s−1 with periods of around 10 min. The detection of such low amplitudes is important for the determination of which magnetic Ap stars pulsate, and which are constant, a distinction important for the understanding of the pulsation driving mechanism in these stars.Monthly Notices of the Royal Astronomical Society Letters 03/2010; 404(1):L104 - L108. -
Article: Strömgren Photometry of Galactic Globular Clusters. II. Metallicity Distribution of Red Giants in ω Centauri
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ABSTRACT: We present new intermediate-band Strömgren photometry based on more than 300 u, v, b, y images of the Galactic globular cluster ω Cen. Optical data were supplemented with new multiband near-infrared (NIR) photometry (350 J, H, Ks images). The final optical-NIR catalog covers a region of more than 20 × 20 arcmin squared across the cluster center. We use different optical-NIR color-color planes together with proper-motion data available in the literature to identify candidate cluster red-giant (RG) stars. By adopting different Strömgren metallicity indices, we estimate the photometric metallicity for 4000 RGs, the largest sample ever collected. The metallicity distributions show multiple peaks ([Fe/H]phot = –1.73 ± 0.08, –1.29 ± 0.03, –1.05 ± 0.02, –0.80 ± 0.04, –0.42 ± 0.12, and –0.07 ± 0.08 dex) and a sharp cutoff in the metal-poor (MP) tail ([Fe/H]phot –2 dex) that agree quite well with spectroscopic measurements. We identify four distinct subpopulations, namely, MP ([Fe/H] ≤ –1.49), metal-intermediate (MI; –1.49 < [Fe/H] ≤ –0.93), metal-rich (MR; –0.95 < [Fe/H] ≤ –0.15), and solar metallicity ([Fe/H] 0). The last group includes only a small fraction of stars (~8% ± 5%) and should be confirmed spectroscopically. Moreover, using the difference in metallicity based on different photometric indices, we find that the 19% ± 1% of RGs are candidate CN-strong stars. This fraction agrees quite well with recent spectroscopic estimates and could imply a large fraction of binary stars. The Strömgren metallicity indices display a robust correlation with α-elements ([Ca+Si/H]) when moving from the MI to the MR regime ([Fe/H] –1.7 dex).The Astrophysical Journal 11/2009; 706(2):1277. · 6.02 Impact Factor -
Article: Stromgren photometry of Galactic Globular Clusters. II Metallicity distribution of red giants in Omega Centauri
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ABSTRACT: We present new intermediate-band Stroemgren photometry based on more than 300 u,v,b,y images of the Galactic globular cluster Omega Cen. Optical data were supplemented with new multiband near-infrared (NIR) photometry (350 J,H,K_s images). The final optical-NIR catalog covers a region of more than 20*20 arcmin squared across the cluster center. We use different optical-NIR color-color planes together with proper motion data available in the literature to identify candidate cluster red giant (RG) stars. By adopting different Stroemgren metallicity indices we estimate the photometric metallicity for ~4,000 RGs, the largest sample ever collected. The metallicity distributions show multiple peaks ([Fe/H]_phot=-1.73+/-0.08,-1.29+/-0.03,-1.05+/-0.02,-0.80+/-0.04,-0.42+/-0.12 and -0.07+/-0.08 dex) and a sharp cut-off in the metal-poor tail ([Fe/H]_phot<=-2 dex) that agree quite well with spectroscopic measurements. We identify four distinct sub-populations,namely metal-poor (MP,[Fe/H]<=-1.49), metal-intermediate (MI,-1.49<[Fe/H]<=-0.93), metal-rich (MR,-0.95<[Fe/H]<=-0.15) and solar metallicity (SM,[Fe/H]~0). The last group includes only a small fraction of stars (~8+/-5%) and should be confirmed spectroscopically. Moreover, using the difference in metallicity based on different photometric indices, we find that the 19+/-1% of RGs are candidate CN-strong stars. This fraction agrees quite well with recent spectroscopic estimates and could imply a large fraction of binary stars. The Stroemgren metallicity indices display a robust correlation with alpha-elements ([Ca+Si/H]) when moving from the metal-intermediate to the metal-rich regime ([Fe/H]>-1.7 dex). Comment: 28 pages, 22 figures, accepted on ApJ09/2009; -
Article: A rival for Babcock's star: the extreme 30-kG variable magnetic field in the Ap star HD 75049
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ABSTRACT: The extraordinary magnetic Ap star HD75049 has been studied with data obtained with the ESO VLT and 2.2-m telescopes. Direct measurements reveal that the magnetic field modulus at maximum reaches 30kG. The star shows photometric, spectral and magnetic variability with a rotation period of 4.049d. Variations of the mean longitudinal magnetic field can be described to first order by a centred dipole model with an inclination i = 25 degrees, an obliquity beta = 60 degrees, and a polar field B_p = 42kG. The combination of the longitudinal and surface magnetic field measurements imply a radius of R = 1.7 R sun, suggesting the star is close to the zero-age main sequence. HD75049 displays moderate overabundances of Si, Ti, Cr, Fe and large overabundances of rare earth elements. This star has the second strongest magnetic field of any main sequence star after Babcock's star, HD215441, which it rivals.08/2009; -
Article: A 3-D study of the photosphere of HD99563: I. Pulsation analysis
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ABSTRACT: We have used high-speed spectroscopy of the rapidly oscillating Ap (roAp) star HD99563 to study the pulsation amplitude and phase behaviour of elements in its stratified atmosphere over one 2.91-d rotation cycle. We identify spectral features related to patches in the surface distribution of chemical elements and study the pulsation amplitudes and phases as the patches move across the stellar disk. The variations are consistent with a distorted nonradial dipole pulsation mode. We measure a 1.6 km/s rotational variation in the mean radial velocities of H-alpha and argue that this is the first observation of H-alpha abundance spots caused by He settling through suppression of convection by the magnetic field on an oblique rotator, in support of a prime theory for the excitation mechanism of roAp star pulsation. We demonstrate that HD99563 is the second roAp star to show aspect dependence of blue-to-red running wave line profile variations in Nd iii spots.05/2009; -
Article: A 3D study of the photosphere of HD 99563 – I. Pulsation analysis★
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ABSTRACT: We have used high-speed spectroscopy of the rapidly oscillating Ap (roAp) star HD 99563 to study the pulsation amplitude and phase behaviour of elements in its stratified atmosphere over one 2.91-d rotation cycle. We identify spectral features related to patches in the surface distribution of chemical elements and study the pulsation amplitudes and phases as the patches move across the stellar disc. The variations are consistent with a distorted non-radial dipole pulsation mode. We measure a 1.6 km s−1 rotational variation in the mean radial velocities of Hα and argue that this is the first observation of Hα abundance spots caused by He settling through suppression of convection by the magnetic field on an oblique rotator, in support of a prime theory for the excitation mechanism of roAp star pulsation. We demonstrate that HD 99563 is the second roAp star to show aspect dependence of blue-to-red running wave line profile variations in Nd iii spots.Monthly Notices of the Royal Astronomical Society 05/2009; 396(1):325 - 342. · 4.90 Impact Factor -
Chapter: MAD@VLT: Deep into the Madding Crowd of Omega Centauri
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ABSTRACT: We present deep and accurate Near-Infrared (NIR) photometry of the Galactic Globular Cluster (GC) ω Cen. Data were collected using the Multi-Conjugate Adaptive Optics Demonstrator (MAD) on VLT (ESO). The unprecedented quality of the images provided the opportunity to perform accurate photometry in the central crowded regions. Preliminary results indicate that the spread in age among the different stellar populations in ω Cen is limited.12/2008: pages 67-71; -
Article: On the White Dwarf Cooling Sequence of the Globular Cluster ω Centauri
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ABSTRACT: We present deep and precise photometry (F435W, F625W, F658N) of ω Cen collected with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST). We have identified 6500 white dwarf (WD) candidates, and the ratio of WD to main-sequence (MS) star counts is found to be at least a factor of 2 larger than the ratio of CO-core WD cooling to MS lifetimes. This discrepancy is not explained by the possible occurrence of a He-enhanced stellar population, since the MS lifetime changes by only 15% when changing from a canonical (Y = 0.25) to a He-enhanced composition (Y = 0.42). The presence of some He-core WDs seems able to explain the observed star counts. The fraction of He WDs required ranges from 10% to 80% depending on their mean mass, and it is at least 5 times larger than for field WDs. The comparison in the color-magnitude diagram between theory and observations also supports the presence of He WDs. Empirical evidence indicates that He WDs have been detected in stellar systems hosting a large sample of extreme horizontal branch stars, thus suggesting that a fraction of red giants might avoid the He-core flash.The Astrophysical Journal 12/2008; 673(1):L29. · 6.02 Impact Factor -
Article: On the Anomalous Red Giant Branch of the Globular Cluster ω Centauri
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ABSTRACT: We present three different optical and near-infrared (NIR) data sets for evolved stars in the Galactic globular cluster ω Centauri. The comparison between observations and homogeneous sets of stellar isochrones and zero-age horizontal branches provides two reasonable fits. Both of them suggest that the so-called anomalous branch has a metal-intermediate chemical composition (-1.1 ≤ [Fe/H] ≤ -0.8) and is located ~500 pc beyond the bulk of ω Cen stars. These findings are mainly supported by the shape of the subgiant branch in four different color-magnitude diagrams (CMDs). The most plausible fit requires a higher reddening, E(B - V) = 0.155 versus 0.12, and suggests that the anomalous branch is coeval with the bulk of ω Cen stellar populations to within empirical and theoretical uncertainties. This result is supported by the identification of a sample of faint horizontal-branch (HB) stars that might be connected with the anomalous branch. Circumstantial empirical evidence seems to suggest that the stars in this branch form a clump of stars located beyond the cluster.The Astrophysical Journal 12/2008; 623(2):860. · 6.02 Impact Factor -
Article: The Discovery of More than 2000 White Dwarfs in the Globular Cluster ω Centauri
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ABSTRACT: We present deep multiband (F435W, F625W, and F658N) photometric data of the globular cluster ω Cen collected with the Advanced Camera for Surveys on board the Hubble Space Telescope. We identified in the (F435W - F625W, F435W)-plane more than 2000 white dwarf (WD) candidates using three out of nine available pointings. Such a large sample appears to be in agreement with predictions based on the ratio between WD and horizontal-branch evolutionary lifetimes. We also detected ≈1600 WDs in the (F658N - F625W, F625W)-plane, supporting the evidence that a large fraction of current cluster WDs are Hα-bright.The Astrophysical Journal 12/2008; 621(2):L117. · 6.02 Impact Factor -
Article: Reddening Distribution across the Center of the Globular Cluster ω Centauri
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ABSTRACT: We present new medium-band uvby Strömgren and broadband VI photometry for the central regions of the globular cluster ω Cen. From this photometry we have obtained differential reddening estimates relative to two other globular clusters (M13 and NGC 288) using a metallicity-independent, reddening-free temperature index, [c] ≡ (u-v) - (v-b) - 0.2(b-y), for hot horizontal-branch (HB) stars (Teff ≥ 8500 K). We estimate color excesses of these hot HB stars using optical and near-infrared colors, and we find clumpy extinction variations of almost a factor of 2 within the area of the cluster core. In particular, the greatest density of more highly reddened objects appears to be shifted along the right ascension axis when compared with less reddened ones. These findings complicate photometric efforts to investigate the star formation history of ω Cen.The Astrophysical Journal 12/2008; 634(1):L69. · 6.02 Impact Factor -
Article: A Pulsational Distance to ω Centauri Based on Near-Infrared Period-Luminosity Relations of RR Lyrae Stars
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ABSTRACT: We present new near-infrared (J and K) magnitudes for 114 RR Lyrae stars in the globular cluster ω Centauri (NGC 5139), which we combine with data from the literature to construct a sample of 180 RR Lyrae stars with J and K mean magnitudes on a common photometric system. This is currently the largest such sample in any stellar system. We also present updated predictions for J- and K-band period-luminosity relations for both fundamental and first-overtone RR Lyrae stars, based on synthetic horizontal branch models with metal abundance ranging from Z = 0.0001 to 0.004. By adopting for the ω Cen variables with measured metal abundances an α-element enhancement of a factor of 3 (≈0.5 dex) with respect to iron, we find a true distance modulus μ0 = 13.70 ± 0.06 ± 0.06 (random and systematic errors, respectively), corresponding to a distance d = 5.5 ± 0.03 ± 0.03 kpc. Our estimate is in excellent agreement with the distance inferred for the eclipsing binary OGLEGC 17, but differ significantly from the recent distance estimates based on cluster dynamics and on high-amplitude δ Scuti stars.The Astrophysical Journal 12/2008; 652(1):362. · 6.02 Impact Factor -
Article: On the Relative Distances of ω Centauri and 47 Tucanae
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ABSTRACT: We present precise optical and near-infrared ground-based photometry of two globular clusters (GCs): ω Cen and 47 Tuc. These photometric catalogs are unbiased in the red giant branch (RGB) region close to the tip. We provide new estimates of the RGB tip (TRGB) magnitudes—mI(TRGB) = 9.84 ± 0.05, ω Cen; mI(TRGB) = 9.46 ± 0.06, 47 Tuc—and use these to determine the relative distances of the two GCs. We find that distance ratios based on different calibrations of the TRGB, the RR Lyrae stars, and kinematic distances agree with each other within 1 σ. Absolute TRGB and RR Lyrae distance moduli agree within 0.10-0.15 mag, while absolute kinematic distance moduli are 0.2-0.3 mag smaller. Absolute distances to 47 Tuc based on the zero-age horizontal branch and on the white dwarf fitting agree within 0.1 mag, but they are 0.1-0.3 mag smaller than TRGB and RR Lyrae distances.The Astrophysical Journal 12/2008; 686(2):L87. · 6.02 Impact Factor -
Article: On the spectroscopic nature of the cool evolved Am star HD 151878★
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ABSTRACT: Recently, Tiwari, Chaubey & Pandey detected the bright component of the visual binary HD 151878 to exhibit rapid photometric oscillations through a Johnson B filter with a period of 6 min (2.78 mHz) and a high, modulated amplitude up to 22 mmag peak-to-peak, making this star by far the highest amplitude rapidly oscillating Ap (roAp) star known. As a new roAp star, HD 151878 is of additional particular interest as a scarce example of the class in the northern sky, and only the second known case of an evolved roAp star – the other being HD 116114. We used the FIbre-fed Echelle Spectrograph at the Nordic Optical Telescope to obtain high time-resolution spectra at high dispersion to attempt to verify the rapid oscillations. We show here that the star at this epoch is spectroscopically stable to rapid oscillations of no more than a few tens of m s−1. The high-resolution spectra furthermore show the star to be of type Am rather than Ap and we show the star lacks most of the known characteristics for roAp stars. We conclude that this is an Am star that does not pulsate with a 6-min period. The original discovery of pulsation is likely to be an instrumental artefact.Monthly Notices of the Royal Astronomical Society 10/2008; 390(1):257 - 264. · 4.90 Impact Factor -
Article: Relative distances of Omega Centauri and 47 Tucanae
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ABSTRACT: We present precise optical and near-infrared ground-based photometry of two Globular Clusters (GCs): Omega Cen and 47 Tuc. These photometric catalogs are unbiased in the Red Giant Branch (RGB) region close to the tip. We provide new estimates of the RGB tip (TRGB) magnitudes--m_I(TRGB)=9.84+/-0.05, Omega Cen; m_I(TRGB)=9.46+/-0.06, 47 Tuc--and use these to determine the relative distances of the two GCs. We find that distance ratios based on different calibrations of the TRGB, the RR Lyrae stars and kinematic distances agree with each other within one sigma. Absolute TRGB and RR Lyrae distance moduli agree within 0.10--0.15 mag, while absolute kinematic distance moduli are 0.2--0.3 mag smaller. Absolute distances to 47 Tuc based on the Zero-Age-Horizontal-Branch and on the white dwarf fitting agree within 0.1 mag, but they are 0.1--0.3 mag smaller than TRGB and RR Lyrae distances. Comment: 5 pages, 4 figures, accepted for publication by ApJL09/2008; -
Article: A search for rapid pulsations in the magnetic cool chemically peculiar star HD3980
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ABSTRACT: The Ap star HD3980 appears to be a promising roAp candidate based on its fundamental parameters, leading us to search for rapid pulsations with the VLT UV-Visual Echelle Spectrograph (UVES). A precise Hipparcos parallax and estimated temperature of 8100K place HD3980 in the middle of the theoretical instability strip for rapidly oscillating Ap stars, about halfway through its main sequence evolution stage. The star has a strong, variable magnetic field, as is typical of the cool magnetic Ap stars. Dipole model parameters were determined from VLT observations using FORS1. From Doppler shift measurements for individual spectral lines of rare earth elements and the H-alpha line core, we find no pulsations above 20-30 m/s. This result is corroborated by inspection of lines of several other chemical elements, as well as with crosscorrelation for long spectral regions with the average spectrum as a template. Abundances of chemical elements were determined and show larger than solar abundances of rare earth elements. Further, ionisation disequilibria for the first two ionised states of Nd and Pr are detected. We also find that the star has a strong overabundance of manganese, which is typical for much hotter HgMn and other Bp stars. Line profile variability with the rotation period was detected for the majority of chemical species. Comment: Accepted for publication in the Monthly Notices of the Royal Astronomical Society08/2008; -
Article: On the spectroscopic nature of the cool evolved Am star HD151878
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ABSTRACT: Recently, Tiwari, Chaubey, & Pandey (2007) detected the bright component of the visual binary HD151878 to exhibit rapid photometric oscillations through a Johnson B filter with a period of 6 min (2.78 mHz) and a high, modulated amplitude up to 22 mmag peak-to-peak, making this star by far the highest amplitude roAp star known. As a new roAp star, HD151878 is of additional particular interest as a scarce example of the class in the northern sky, and only the second known case of an evolved roAp star - the other being HD 116114. We used the FIES spectrograph at the Nordic Optical Telescope to obtain high time resolution spectra at high dispersion to attempt to verify the rapid oscillations. We show here that the star at this epoch is spectroscopically stable to rapid oscillations of no more than a few tens of m/s. The high-resolution spectra furthermore show the star to be of type Am rather than Ap and we show the star lacks most of the known characteristics for rapidly oscillating Ap stars. We conclude that this is an Am star that does not pulsate with a 6-min period. The original discovery of pulsation is likely to be an instrumental artefact. Comment: Accepted for publication in the Monthly Notices of the Royal Astronomical Society Main Journal07/2008; -
Article: Discovery of 17 new sharp-lined Ap stars with magnetically resolved lines
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ABSTRACT: Chemically peculiar A stars (Ap) are extreme examples of the interaction of atomic element diffusion processes with magnetic fields in stellar atmospheres. The rapidly oscillating Ap stars provide a means for studying these processes in 3D and are at the same time important for studying the pulsation excitation mechanism in A stars. As part of the first comprehensive, uniform, high resolution spectroscopic survey of Ap stars, which we are conducting in the southern hemisphere with the Michigan Spectral Catalogues as the basis of target selection, we report here the discovery of 17 new magnetic Ap stars having spectroscopically resolved Zeeman components from which we derive magnetic field moduli in the range 3 - 30 kG. Among these are 1) the current second-strongest known magnetic A star, 2) a double-lined Ap binary with a magnetic component and 3) an A star with particularly peculiar and variable abundances. Polarimetry of these stars is needed to constrain their field geometries and to determine their rotation periods. We have also obtained an additional measurement of the magnetic field of the Ap star HD 92499. Comment: Accepted for publication by Monthly Notices of the Royal Astronomical Society06/2008; -
Article: MAD@VLT: Deep into the Madding Crowd of Omega Centauri
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ABSTRACT: We present deep and accurate Near-Infrared (NIR) photometry of the Galactic Globular Cluster (GC) Omega Cen. Data were collected using the Multi-Conjugate Adaptive Optics Demonstrator (MAD) on VLT (ESO). The unprecedented quality of the images provided the opportunity to perform accurate photometry in the central crowded regions. Preliminary results indicate that the spread in age among the different stellar populations in Omega Cen is limited. Comment: 6 pages, 3 figures, to appear in the Springer Astrophysics and Space Science Proceedings, "Science with the VLT in the ELT era", ed. A. Moorwood03/2008; -
Article: Relative and absolute calibration for multi-band data collected with the 2.2m ESO/MPI and 1.54m Danish Telescopes
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ABSTRACT: We present the strategies adopted in the relative and absolute calibration of two different data sets: U,B,V,I-band images collected with the Wide Field Imager (WFI) mosaic camera mounted on the 2.2m ESO/MPI Telescope and u,v,b,y Stroemgren images collected with the 1.54m Danish Telescope (ESO, La Silla). In the case of the WFI camera we adopted two methods for the calibration, one for images collected before 2002, with the ESO filters U/38_ESO841 and B/99_ESO842, and a different one for data secured after 2002, with the filters U/50_ESO877 and B/123_ESO878. The positional and color effects turned out to be stronger for images collected with the old filters. The eight WFI chips of these images were corrected one by one, while in the case of images secured with the new filters, we corrected the entire mosaic in a single step. In the case of the Danish data set, we compared point-spread function (PSF) and aperture photometry for each frame, finding a trend in both the X and Y directions of the chip. The corrections resulted in a set of first and second order polynomials to be applied to the instrumental magnitudes of each individual frame as a function of the star position.02/2008;
Top co-authors
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Institutions
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2007–2010
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University of Central Lancashire
Preston, ENG, United Kingdom
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2008
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The Royal Library of Belgium
Brussels, BRU, Belgium
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2005–2008
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Royal Observatory of Belgium
Brussels, BRU, Belgium
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