arXiv subjects

F. P. Pijpers

Publications and source records attributed to F. P. Pijpers.

14 recordsLinked to original sources

arXiv2021

Asteroseismogyrometry of low-mass red giants I. The SOLA inversion method

During the past 10 years the unprecedented quality and frequency resolution of asteroseismic data provided by space photometry has revolutionized the study of red-giant stars providing us with the possibility to probe the interior of thousands of these targets. Our aim is to present an asteroseismic tool which allows to determine the total angular momentum of stars, without a priori inference of their internal rotational profile. We adopt and adapt to red giants the asteroseismic inversion technique developed for the case of the Sun. The method has been tested assuming different artificial sets of data, including also modes with harmonic degree l> 1. We estimate with an accuracy of 14.5% the total angular momentum of the red-giant star KIC 4448777 observed by Kepler during the first four consecutive years of operation.} Our results indicate that the measurement of the total angular momentum of red-giant stars can be determined with a fairly high precision by means of asteroseismology by using a small set of rotational splittings of only dipolar modes and that our method, based on observations of stellar pulsations, provides a powerful mean for testing and modeling transport of angular momentum in stars.

astro-ph.SR↗

arXiv2008

Solar-like oscillations in the G8 V star tau Ceti

We used HARPS to measure oscillations in the low-mass star tau Cet. Although the data were compromised by instrumental noise, we have been able to extract the main features of the oscillations. We found tau Cet to oscillate with an amplitude that is about half that of the Sun, and with a mode lifetime that is slightly shorter than solar. The large frequency separation is 169 muHz, and we have identified modes with degrees 0, 1, 2, and 3. We used the frequencies to estimate the mean density of the star to an accuracy of 0.45% which, combined with the interferometric radius, gives a mass of 0.783 +/- 0.012 M_sun (1.6%).

astro-ph↗

arXiv2008

Asteroseismology and Interferometry

Asteroseismology provides us with a unique opportunity to improve our understanding of stellar structure and evolution. Recent developments, including the first systematic studies of solar-like pulsators, have boosted the impact of this field of research within Astrophysics and have led to a significant increase in the size of the research community. In the present paper we start by reviewing the basic observational and theoretical properties of classical and solar-like pulsators and present results from some of the most recent and outstanding studies of these stars. We centre our review on those classes of pulsators for which interferometric studies are expected to provide a significant input. We discuss current limitations to asteroseismic studies, including difficulties in mode identification and in the accurate determination of global parameters of pulsating stars, and, after a brief review of those aspects of interferometry that are most relevant in this context, anticipate how interferometric observations may contribute to overcome these limitations. Moreover, we present results of recent pilot studies of pulsating stars involving both asteroseismic and interferometric constraints and look into the future, summarizing ongoing efforts concerning the development of future instruments and satellite missions which are expected to have an impact in this field of research.

astro-ph↗

arXiv2006

Performance metrics

Scientific output varies between research fields and between disciplines within a field such as astrophysics. Even in fields where publication is the primary output, there is considerable variation in publication and hence in citation rates. Data from the Smithsonian/NASA Astrophysics Data System is used to illustrate this problem and argue against a "one size fits all" approach to performance metrics, especially over the short time-span covered by the Research Assessment Exercise (soon underway in the UK).

astro-ph↗

arXiv2006

Triple correlations in local helioseismology

A central step in time-distance local helioseismology techniques is to obtain travel times of packets of wave signals between points or sets of points on the visible surface. Standard ways of determining group or phase travel times involve cross-correlating the signal between locations at the solar photosphere and determining the shift of the envelope of this cross correlation function, or a zero crossing, using a standard wavelet or a reference wave packet. Here a novel method is described which makes use of triple correlations, i.e. cross-correlating signal between three locations. By using an average triple correlation as reference, differential travel times can be extracted in a straightforward manner.

astro-ph↗

arXiv2005

Probing Turbulence with Infrared Observations in OMC1

A statistical analysis is presented of the turbulent velocity structure in the Orion Molecular Cloud at scales ranging from 70 AU to 30000 AU. Results are based on IR Fabry-Perot interferometric observations of shock and photon-excited H2 in the K-band S(1) v=1-0 line at 2.121micron and refer to the dynamical characteristics of warm perturbed gas. Observations establish that the Larson size-linewidth relation is obeyed to the smallest scales studied here extending the range of validity of this relationship by nearly 2 orders of magnitude. The velocity probability distribution function (PDF) is constructed showing extended exponential wings, providing evidence of intermittency, further supported by the skewness and kurtosis of the velocity distribution. Variance and kurtosis of the PDF of velocity differences are constructed as a function of lag. The variance shows an approximate power law dependence on lag, with exponent significantly lower than the Kolmogorov value, and with deviations below 2000AU which are attributed to outflows and possibly disk structures associated with low mass star formation within OMC1. The kurtosis shows strong deviation from a gaussian velocity field, providing evidence of velocity correlations at small lags. Results agree accurately with semi-empirical simulations in Eggers & Wang (1998). In addition, 170 individual H2 emitting clumps have been analysed with sizes between 500 and 2200 AU. These show considerable diversity with regard to PDFs and variance functions. Our analysis constitutes the first characterization of the turbulent velocity field at the scale of star formation and provide a dataset which models of star-forming regions should aim to reproduce.

astro-ph↗

arXiv2000

An optical time-delay estimate for the double gravitational lens system B1600+434

We present optical I-band light curves of the gravitationally lensed double QSO B1600+434 from observations obtained at the Nordic Optical Telescope (NOT) between April 1998 and November 1999. The photometry has been performed by simultaneous deconvolution of all the data frames, involving a numerical lens galaxy model. Four methods have been applied to determine the time delay between the two QSO components, giving a mean estimate of Δ_t = 51+/-4 days (95% confidence level). This is the fourth optical time delay ever measured. Adopting a Omega=0.3, Lambda=0 Universe and using the mass model of Maller et al. (2000), this time-delay estimate yields a Hubble parameter of H_0=52 (+14, -8) km s^-1 Mpc^-1 (95% confidence level) where the errors include time-delay as well as model uncertainties. There are time-dependent offsets between the two (appropriately shifted) light curves that indicate the presence of external variations due to microlensing.

astro-ph↗

arXiv1999

Unbiased image reconstruction as an inverse problem

An unbiased method for improving the resolution of astronomical images is presented. The strategy at the core of this method is to establish a linear transformation between the recorded image and an improved image at some desirable resolution. In order to establish this transformation only the actual point spread function and a desired point spread function need be known. Any image actually recorded is not used in establishing the linear transformation between the recorded and improved image. This method has a number of advantages over other methods currently in use. It is not iterative which means it is not necessary to impose any criteria, objective or otherwise, to stop the iterations. The method does not require an artificial separation of the image into ``smooth'' and ``point-like'' components, and thus is unbiased with respect to the character of structures present in the image. The method produces a linear transformation between the recorded image and the deconvolved image and therefore the propagation of pixel-by-pixel flux error estimates into the deconvolved image is trivial. It is explicitly constrained to preserve photometry.

astro-ph↗

arXiv1998

Helioseismic determination of the solar gravitational quadrupole moment

One of the most well-known tests of General Relativity (GR) results from combining measurements of the anomalous precession of the orbit of Mercury with a determination of the gravitational quadrupole moment of the Sun J_2. The latter can be done by inference from an integral relation between J_2 and the solar internal rotation. New observational data of high quality obtained from the Solar Heliospheric Satellite (SoHO) and from the Global Oscillations Network Group (GONG), allow the determination of the internal rotation velocity of the Sun as a function of radius and latitude with unprecedented spatial resolution and accuracy. As a consequence, a number of global properties of the Sun can also be determined with much higher accuracy, notably the gravitational quadrupole moment of the Sun. The anomalous precession of the orbit of Mercury is primarily due to GR effects but there are classical corrections the largest of which is that due to J_2. It is shown here that the data are currently consistent with the predictions of GR.

astro-ph↗

arXiv1997

The residence time of dust grains in turbulent molecular clouds

The residence time of dust grains inside turbulent molecular clouds is calculated. Earlier estimates of this time by Boland & de Jong (1982) have been criticized by Prasad et al. (1987) for not taking into account properly the turbulent character of the velocity field in molecular clouds which implies that the trajectory of a dust grain should be treated as a random walk. Taking some minimal assumptions regarding the turbulent velocity field a time scale is derived that depends on the Reynolds number of the flow. The near proportionality with Reynolds number of this time scale results in a much longer time scale over which dust grains will remain inside a molecular cloud.

astro-ph↗

arXiv1995

A modified R1 X R1 method for helioseismic rotation inversions

We present an efficient method for two dimensional inversions for the solar rotation rate using the Subtractive Optimally Localized Averages (SOLA) method and a modification of the R1 X R1 technique proposed by Sekii (1993). The SOLA method is based on explicit construction of averaging kernels similar to the Backus-Gilbert method. The versatility and reliability of the SOLA method in reproducing a target form for the averaging kernel, in combination with the idea of the R1 X R1 decomposition, results in a computationally very efficient inversion algorithm. This is particularly important for full 2-D inversions of helioseismic data in which the number of modes runs into at least tens of thousands.

astro-ph↗

arXiv1995

Inversions in astronomy and the SOLA method

This paper was presented at the Institute for Mathematics and its Applications workshop "Inverse problems in wave propagation" and will appear in the series IMA volumes (Springer). A brief overview of applications of inversions within astronomy is presented and also an inventory of techniques commonly in use. Most of this paper is focussed on the method of Subtractive Optimally Localized Averages (SOLA) which is an adaptation of the Backus and Gilbert method. This method was originally developed for use in helioseismology where the Backus and Gilbert method is computationally too slow. Since then it has also been applied to the problem of reverberation mapping of active galactic nuclei and the differences between this inverse problem and the ones of helioseismology are also discussed.

astro-ph↗

arXiv1994

On the propagation of sound waves in a stellar wind traversed by periodic strong shocks

It has been claimed that in stellar winds traversed by strong shocks the mechanism for driving the wind by sound wave pressure cannot operate because sound waves cannot propagate past the shocks. It is shown here that sound waves can propagate through shocks in one direction and that this is a sufficient condition for the sound wave pressure mechanism to work. A strong shock amplifies a sound wave passing through it and can drag the sound wave away from the star. It is immaterial for the sound wave pressure gradient that the sound wave vector points towards the star. Since the strong shocks drag the sound waves away, the star itself is the source for the sound waves propagating towards it.

astro-ph↗

arXiv1994

Short time scale monitoring of SiO sources

We present the results of a short time scale monitoring of SiO maser emission (v=1 J=1-0 transition) in four known strong sources. These sources were monitored nightly for a period of about a month. The aim of these observations is to investigate the possible presence of variations in the maser lines on time scales of a few days to weeks, due to sound waves propagating out from the central star. If sound waves are responsible for the mass loss of certain cool giants, as suggested by Pijpers and Hearn (1989) and Pijpers and Habing (1989), local variations in density and relative velocity are expected just above the stellar photosphere. These could give rise to variations in any narrow spectral line formed in this region, and therefore in particular in the SiO maser lines. Our observations indicate that variations in the line shape (leading to relative changes in the intensity of about 20\%) occur in the SiO emission of Mira type stars, within short time scales of 10-20 days. The main component of the profile variability is consistent with a displacement of the velocity centroid of the dominant maser peaks, by about 1 km s$^{-1}$ in the average. Apparent variations in the total line flux were also found, but could be partially due to calibration uncertainties.

astro-ph↗