arXiv subjects

H. M. Maitzen

Publications and source records attributed to H. M. Maitzen.

30 records · Page 2Linked to original sources

arXiv2005

On the formation and evolution of magnetic chemically peculiar stars in the solar neighborhood

In order to put strict observational constraints on the evolutionary status of the magnetic chemically peculiar stars (CP2) of the upper main sequence, we have investigated a well established sample of galactic field CP2 objects within a radius of 200pc from the Sun in the (X,Y) plane. In total, 182 stars with accurate parallax measurements from the Hipparcos satellite were divided into Si, SiCr and SrCrEu subgroups based on classification resolution data from the literature. Primarily, it was investigated if the CP2 phenomenon occurs at very early stages of the stellar evolution, significantly before these stars reach 30% of their life-time on the main sequence. This result is especially important for theories dealing with stellar dynamos, angular momentum loss during the pre- as well as main sequence and stellar evolutionary codes for CP2 stars. For the calibration of the chosen sample, the well-developed framework of the Geneva 7-color and Stromgren uvbybeta photometric system was used. We are able to show that the CP2 phenomenon occurs continuously at the zero age main sequence for masses between 1.5 and 4.5M(sun}. The magnetic field strengths do not vary significantly during the evolution towards the terminal age main sequence. Only the effective temperature and magnetic field strength seem to determine the kind of peculiarity for those stars. We found several effects during the evolution of CP2 stars at the main sequence, i.e. there are two "critical" temperatures where severe changes take place. There is a transition between Si, SiCr and SrCrEu stars at 10000K whereas a significant decrease to almost zero of evolved SrCrEu objects with masses below 2.25M(sun) at 8000K occurs.

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arXiv2004

New Variable Stars in Open Clusters I: Methods and Results for 20 Open Clusters

We present high precision CCD photometry of 1791 objects in 20 open clusters with an age from 10 Myr to 1 Gyr. These observations were performed within the Delta a photometric system which is primarily used to detect chemically peculiar stars of the upper main sequence. Time bases range between 30 minutes and up to 60 days with data from several nights. We describe the way of time series analysis reaching a detection limit of down to 0.006 mag. In total, we have detected 35 variable objects from which four are not members of their corresponding clusters. The variables cover the entire Hertzsprung-Russell-diagram, hence they are interesting targets for follow-up observations.

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arXiv2003

CCD photometric search for peculiar stars in open clusters. V. NGC 2099, NGC 3114, NGC 6204, NGC 6705 and NGC 6756

We have investigated 1008 objects in the area of five intermediate age open clusters (NGC 2099, NGC 3114, NGC 6204, NGC 6705 and NGC 6756) via the narrow band Delta a-system. The detection limit for photometric peculiarity is very low (always less than 0.009mag) due to the high number of individual frames used (193 in total). We have detected six peculiar objects in NGC 6705 and NGC 6756 from which one in the latter is almost certainly an unreddened late type foreground star. The remaining five stars are probably cluster members and bona fide chemically peculiar objects (two are lambda Bootis type candidates). Furthermore, we have investigated NGC 3114, a cluster for which already photoelectric Delta a-measurements exist. A comparison of the CCD and photoelectric values shows very good agreement. Again, the high capability of our CCD Delta a-photometric system to sort out true peculiar objects together with additional measurements from broad or intermediate band photometry is demonstrated.

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arXiv2003

Theoretical isochrones for the Delta a photometric system

We have calculated theoretical isochrones for the photometric Delta a system to derive astrophysical parameters such as the age, reddening and distance modulus for open clusters. The Delta a system samples the flux depression at 520 nm which is highly efficient to detect chemically peculiar (CP) objects of the upper main sequence. The evolutionary status of CP stars is still a matter of debate and very important to test, for example, the dynamo and diffusion theories. In fact, the dynamo or fossil origin of the magnetic fields present in this kind of stars it still not clear. Using the stellar evolutionary models by Claret (1995), a grid of isochrones with different initial chemical compositions for the Delta a system was generated. The published data of 23 open clusters were used to fit these isochrones with astrophysical parameters (age, reddening and distance modulus) from the literature. As an additional test, isochrones with the same parameters for Johnson UBV data of these open clusters were also considered. The fits show a good agreement between the observations and the theoretical grid. We find that the accuracy of fitting isochrones to Delta a data without the knowledge of the cluster parameters is between 5 and 15%.

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arXiv2003

On the evolutionary status of chemically peculiar stars of the upper main sequence

We present further evidence that the magnetic chemically peculiar stars (CP2) of the upper main sequence already occur at very early stages of the stellar evolution, significantly before they reach 30% of their life-time on the main sequence. This result is especially important for models dealing with dynamo theories, angular momentum loss during the pre- as well as main sequence and evolutionary calculations for CP2 stars. Results from the literature either derived for objects in the Hyades and the UMa cluster or from the Hipparcos mission contradict each other. A way out of this dilemma is to investigate young open clusters with known ages and accurate distances (error < 10%), including CP2 members. Up to now, four open clusters fulfill these requirements: IC 2391, IC 2602, NGC 2451A and NGC 2516. In total, 13 CP2 stars can be found within these clusters. We have used the measurements and calibrations of the Geneva 7-color photometric system to derive effective temperatures and luminosities. Taking into account the overall metallicity of the individual clusters, isochrones and evolutionary tracks were used to estimate ages and masses for the individual objects. The derived ages (between 10 and 140 Myr) are well in line with those of the corresponding clusters and further strengthen the membership of the investigated CP2 stars.

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arXiv2003

The 5200A flux depression of chemically peculiar stars: I. Synthetic Delta a photometry - the normality line

The Delta a photometric system provides an efficient observational method to identify and distinguish magnetic and several other types of chemically peculiar (CP) stars of spectral types B to F from other classes of stars in the same range of effective temperatures. We have developed a synthetic photometric system which can be used to explore the capability of model atmospheres with individual element abundances to predict photometric Delta a magnitudes which measure the extent of the flux depression around 5200A found in different types of CP stars. In this first paper, we confirm the observed dependency of the a-index as a function of various colour indices sensitive to the effective temperature of stars as well as its average scatter expected from surface gravity variations within the main sequence band. The behaviour of the so-called normality line of Delta a systems used in photometric observations of CP stars is well reproduced. The metallicity dependence of the normality line of the Delta a system was computed for several grids of model atmospheres where the abundances of elements heavier than He had been scaled +-0.5 dex with respect to the solar value. We estimate a lowering of Delta a magnitudes for CP stars within the Magellanic Clouds by approximately -3 mmag relative to those in the solar neighbourhood assuming an average metallicity of [Fe/H]= -0.5 dex. Using these results on the metallicity bias of the Delta a system we find the observational systems in use suitable to identify CP stars in other galaxies or distant regions of our own galaxy and capable to provide data samples on a statistically meaningful basis. In turn, the synthetic system is suitable to test the performance of model atmospheres for CP stars. This work will be presented in follow-up papers of this series.

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arXiv2003

Stroemgren uvby photometry of the open clusters NGC 6192 and NGC 6451

We have investigated the two open clusters NGC 6192 and NGC 6451 for which widely different reddening values and thus ages and distances can be found in the literature via Stroemgren uvby photometry. Our measurements allow to disentangle the apparent discrepancies from the literature and to derive new accurate values. From appropriate calibrations we find that the overall abundance for NGC 6192 is about solar whereas a subsolar value for NGC 6451 was estimated. From two previous reported photometrically candidate CP stars, one within NGC 6192 shows Stroemgren indices typical for a B8Si star whereas the other object of NGC 6451 is most probably a foreground G-type star.

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arXiv2002

Rotation of stars in NGC 6134: a comparison of Delta Scuti stars and non-variable stars

We present results of spectroscopic observations of selected stars in the southern open cluster NGC 6134. We have determined the rotational velocities of the six known Delta Scuti Stars in NGC 6134 as well as several other non-variable stars with similar colour temperature in order to investigate if v sin i and variability is somehow connected: we find no such correlation. We also compare the distribution of v sin i of Delta Scuti Stars and non-variable stars with four other well-studied open clusters to look for any systematic behaviour, but we find no conclusive evidence for v sin i and variability to be connected. We have also used the spectra to carry out an abundance analysis of the Delta Scuti Stars in NGC 6134 to confirm the high metal content of the cluster. We find [Fe/H] = +0.38 +/- 0.05 which is in agreement with the result obtained from Stromgren photometry. We also present Delta-a photometry of the cluster, but we find no chemical peculiar stars based on this index.

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arXiv2001

Origin and possible birthplace of the extreme runaway star HIP 60350

Using the recently determined spatial velocity components of the extreme runaway star HIP 60350 and a gravitation potential model of the Galaxy, we integrate the orbit of HIP 60350 back to the plane of the Galaxy. In this way, a possible location of the formation of the star is determined. We estimate the uncertainty of the result due to the uncertainties of the gravitational potential model and the errors in the spatial velocity components. The place of birth lies (within the errors) near the position of the open cluster NGC 3603. However, the ejection event which occured about 20 Myr ago is in contradiction with the cluster mean age of 3-4 Myr. We suggest that it occured at an earlier phase in sequential star formation in that region. We discuss also ejection mechanisms. Due to the rather high mass of the star (about $5 \Msol$), the most probable model is that of dynamical ejection.

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arXiv2000

Ages and Metallicities of Fornax Dwarf Ellipticals

Narrow band photometry is presented on 27 dwarf ellipticals in the Fornax cluster. Calibrated with Galactic globular cluster data and spectrophotometric population models, the colors indicated that dwarf ellipticals have a mean [Fe/H] of -1.00+/-0.28 ranging from -1.6 to -0.4. The mean age of dwarf ellipticals, also determined photometrically, is estimated at 10+/-1 Gyrs compared to 13 Gyrs for bright Fornax ellipticals. Comparison of our metallicity color and Mg_2 indices demonstrates that the [Mg/Fe] ratio is lower in dwarf ellipticals than their more massive cousins, which is consistent with a longer duration of initial star formation to explain their younger ages. There is a increase in dwarf metallicity with distance from the Fornax cluster center where core galaxies are, on average, 0.5 dex more metal-poor than halo dwarfs. In addition, we find the halo dwarfs are younger in mean age compared to core dwarfs. One possible explanation is that the intracluster medium ram pressure strips the gas from dwarf ellipticals halting star formation (old age) and stopping enrichment (low metallicity) as they enter the core.

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arXiv1998

Statistics on the spectral classification of CP2 stars in the Southern Sky

A number of about 1500 spectroscopically classified CP2 stars in the southern sky (delta =-90 to -12 degrees) was extracted from the Michigan Catalogue (Vols. I - IV). This sample was compared with the classification from Bidelman & Mc Connell (1973). We confirmed the spectral classification with the known photometric peculiarity indices in the Geneva system and in the 3-filter Delta a system (Maitzen 1976). 10% of these stars show discordance between their respective types from the Michigan and Bidelman catalogues. Several objects were measured with a CCD in the Delta a system in spring 1995. Eight stars are peculiar in Delta a. Furthermore, we have investigated the galactic distribution of all programme stars. We conclude that the distribution resembles the one of early-type stars, where the hotter (= Silicon) stars are more concentrated towards the galactic plane than the cooler (= Strontium) objects.

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arXiv1998

CP2 stars in clusters: deep Delta a-photometry

The search for chemically peculiar (CP) stars in open clusters using photoelectric photometry sampling the presence of the characteristic flux depression feature at 5200A via the Delta a-system (Maitzen 1976) has so far delivered data for objects usually no more distant than 1000 pc from the Sun. If one intends to study the presence of CP stars at larger distances from the Sun, classical photometry has to be replaced by CCD photometry. For the first time, our investigation presents the results of CCD-photometry in the Delta a-system for a rich open cluster which is at a distance clearly beyond hitherto studied objects, Melotte 105 (2 kpc, log age = 8.5). Comparison with published uvby-photometry yields the calibration of the colour index g_1-y of our system, which is necessary for deriving the peculiarity index Delta a. For this we achieve an average accuracy of 0.007 mag. Six objects with only marginally peculiar Delta a-values were found, but spectroscopic and additional photometric evidence is needed to substantiate their peculiarity.

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