Distances and absolute magnitudes from trigonometric parallaxes

We first review the current knowledge of Hipparcos systematic and random errors, in particular small-scale correlations. Then, assuming Gaussian parallax errors and using examples from the recent Hipparcos literature, we show how random errors may be misinterpreted as systematic errors, or transformed into systematic errors. Finally we summarise how to get unbiased estimates of absolute magnitudes and distances, using either Bayesian or non-parametrical methods. These methods may be applied to get either mean quantities or individual estimates. In particular, we underline the notion of astrometry-based luminosity, which avoids the truncation biases and allows a full use of Hipparcos samples.Comment: 20 pages, 8 figures, Invited paper in Haguenau Colloquium "Harmonizing Cosmic Distance Scales in a Post-Hipparcos Era", 14-16/09/98, to appear in ASP Conf. Series, D. Egret and A. Heck ed

An updated maximum likelihood approach to open cluster distance determination

An improved method for estimating distances to open clusters is presented and applied to Hipparcos data for the Pleiades and the Hyades. The method is applied in the context of the historic Pleiades distance problem, with a discussion of previous criticisms of Hipparcos parallaxes. This is followed by an outlook for Gaia, where the improved method could be especially useful. Based on maximum likelihood estimation, the method combines parallax, position, apparent magnitude, colour, proper motion, and radial velocity information to estimate the parameters describing an open cluster precisely and without bias. We find the distance to the Pleiades to be $120.3 \pm 1.5$ pc, in accordance with previously published work using the same dataset. We find that error correlations cannot be responsible for the still present discrepancy between Hipparcos and photometric methods. Additionally, the three-dimensional space velocity and physical structure of Pleiades is parametrised, where we find strong evidence of mass segregation. The distance to the Hyades is found to be $46.35\pm 0.35$ pc, also in accordance with previous results. Through the use of simulations, we confirm that the method is unbiased, so will be useful for accurate open cluster parameter estimation with Gaia at distances up to several thousand parsec.Comment: 14 pages, 8 figures, 6 tables, 3 appendices. Accepted in A&

Period-Luminosity-Colour distribution and classification of Galactic oxygen-rich LPVs

The absolute K magnitudes and kinematic parameters of about 350 oxygen-rich Long-Period Variable stars are calibrated, by means of an up-to-date maximum-likelihood method, using Hipparcos parallaxes and proper motions together with radial velocities and, as additional data, periods and V-K colour indices. Four groups, differing by their kinematics and mean magnitudes, are found. For each of them, we also obtain the distributions of magnitude, period and de-reddened colour of the base population, as well as de-biased period-luminosity-colour relations and their two-dimensional projections. The SRa semiregulars do not seem to constitute a separate class of LPVs. The SRb appear to belong to two populations of different ages. In a PL diagram, they constitute two evolutionary sequences towards the Mira stage. The Miras of the disk appear to pulsate on a lower-order mode. The slopes of their de-biased PL and PC relations are found to be very different from the ones of the Oxygen Miras of the LMC. This suggests that a significant number of so-called Miras of the LMC are misclassified. This also suggests that the Miras of the LMC do not constitute a homogeneous group, but include a significant proportion of metal-deficient stars, suggesting a relatively smooth star formation history. As a consequence, one may not trivially transpose the LMC period-luminosity relation from one galaxy to the other.Comment: 13 pages, 23 figures. Accepted for publication in A&A

Hunting for open clusters in \textit{Gaia} DR2: the Galactic anticentre

The Gaia Data Release 2 (DR2) provided an unprecedented volume of precise astrometric and excellent photometric data. In terms of data mining the Gaia catalogue, machine learning methods have shown to be a powerful tool, for instance in the search for unknown stellar structures. Particularly, supervised and unsupervised learning methods combined together significantly improves the detection rate of open clusters. We systematically scan Gaia DR2 in a region covering the Galactic anticentre and the Perseus arm $(120 \leq l \leq 205$ and $-10 \leq b \leq 10)$, with the goal of finding any open clusters that may exist in this region, and fine tuning a previously proposed methodology successfully applied to TGAS data, adapting it to different density regions. Our methodology uses an unsupervised, density-based, clustering algorithm, DBSCAN, that identifies overdensities in the five-dimensional astrometric parameter space $(l,b,\varpi,\mu_{\alpha^*},\mu_{\delta})$ that may correspond to physical clusters. The overdensities are separated into physical clusters (open clusters) or random statistical clusters using an artificial neural network to recognise the isochrone pattern that open clusters show in a colour magnitude diagram. The method is able to recover more than 75% of the open clusters confirmed in the search area. Moreover, we detected 53 open clusters unknown previous to Gaia DR2, which represents an increase of more than 22% with respect to the already catalogued clusters in this region. We find that the census of nearby open clusters is not complete. Different machine learning methodologies for a blind search of open clusters are complementary to each other; no single method is able to detect 100% of the existing groups. Our methodology has shown to be a reliable tool for the automatic detection of open clusters, designed to be applied to the full Gaia DR2 catalogue.Comment: 8 pages, accepted by Astronomy and Astrophysics (A&A) the 14th May, 2019. Tables 1 and 2 available at the CD

Long Period Variable Stars: galactic populations and infrared luminosity calibrations

In this paper HIPPARCOS astrometric and kinematic data are used to calibrate both infrared luminosities and kinematical parameters of Long Period Variable stars (LPVs). Individual absolute K and IRAS 12 and 25 luminosities of 800 LPVs are determined and made available in electronic form. The estimated mean kinematics is analyzed in terms of galactic populations. LPVs are found to belong to galactic populations ranging from the thin disk to the extended disk. An age range and a lower limit of the initial mass is given for stars of each population. A difference of 1.3mag in K for the upper limit of the Asymptotic Giant Branch is found between the disk and old disk galactic populations, confirming its dependence on the mass in the main sequence. LPVs with a thin envelope are distinguished using the estimated mean IRAS luminosities. The level of attraction (in the classification sense) of each group for the usual classifying parameters of LPVs (variability and spectral types) is examined.Comment: Accepted for publication in A&A (May 2001). 13 page

Dynamical streams in the solar neighbourhood

The true nature of the Hyades and Sirius superclusters is still an open question. In this contribution, we confront Eggen's hypothesis that they are cluster remnants with the results of a kinematic analysis of more than 6000 K and M giants in the solar neighbourhood. This analysis includes new radial velocity data from a large survey performed with the Coravel spectrometer, complemented by Hipparcos parallaxes and Tycho-2 proper motions (Famaey et al. 2004). A maximum-likelihood method, based on a bayesian approach, has been applied to the data, in order to make full use of all the available data (including less precise parallaxes) and to derive the properties of the different kinematic subgroups. Two such subgroups can be identified with the Hyades and Sirius superclusters. Stars belonging to them span a very wide range of age, which is difficult to account for in Eggen's scenario. These groups are thus most probably "dynamical streams" related to the dynamical perturbation by spiral waves rather than to cluster remnants. In this scenario, the Hyades and Ursa Major clusters just happen to be in the Hyades and Sirius streams, which are purely dynamical features that have nothing to do with the remnants of more massive primordial clusters. This mechanism could be the key to understanding the presence of an old metal-rich population, and of many exoplanetary systems in our neighbourhood. Moreover, a strong spiral pattern seems to be needed in order to yield such prominent streams. Since spiral structure is usually baryonic, this would leave very little room for dark matter. This may be an indication that the era of the dark-matter paradigm explaining the dynamics of the Galaxy may come to an end, and is being superseded by modified gravity.Comment: 5 pages, 1 figure, to appear in The Three Dimensional Universe with GAIA, eds M. Perryman & C. Turo

Stellar and circumstellar evolution of long period variable stars

In a first paper, HIPPARCOS astrometric and kinematic data were used to calibrate both infrared K and IRAS luminosities at the same time as kinematic parameters of Long Period Variable stars (LPVs). Individual estimated absolute magnitudes and a probabilistic assignation to galactic populations were deduced from these calibrations for each LPV of our sample. Here we propose a scenario of simultaneous stellar and circumstellar evolution according to the galactic populations. The transitory states of S and Tc stars allow us to confirm the location of the first dredge-up at $M_{bol}=-3.5$. There is also evidence suggesting that a previous enrichment in s-elements from a more evolved companion may accelerate the evolution along the AGB. The possible evolution to OH LPVs is included in this scenario, and any of these stars may have a mass at the limit of the capability for a C enrichment up to C/O > 1. A list of bright massive LPVs with peculiar envelope and luminosity properties is proposed as Hot Bottom Burning candidates. The He-shell flash star, R Cen, is found to be exceptionally bright and could become, before leaving the AGB, a C-rich LPV brighter than the usual luminosity limit of carbon stars.Comment: Accepted for publication in Astronomy & Astrophysic

Absolute magnitudes and kinematics of CP stars from Hipparcos data

The position in the HR diagram and the kinematic characteristics of different kinds of CP stars of the upper main sequence are obtained using the LM method (Luri et al., 1996). Most of the CP stars are main sequence stars occupying the whole width of the sequence. From a kinematic point of view, they belong to the young disk population (ages < 1.5 Gyr). It has also been found that, on kinematic grounds, the behaviour of lambda Bootis stars is similar to the one observed for normal stars of the same spectral range. On the other hand, roAp and noAp stars show the same kinematic characteristics. The peculiar velocity distribution function has been decomposed into a sum of three dimensional gaussians and the presence of Pleiades, Sirius and Hyades moving groups has been clearly established. Finally, a small number of CP stars are found to be high-velocity objects.Comment: 8 pages, 1 figure, to appear in: Proc. of the 26th workshop of the European Working Group on CP stars, eds. P. North, A. Schnell and J. Ziznovsky, Contrib. Astr. Obs. Skalnate Pleso Vol. 27, No