arXiv subjects

H. M. Maitzen

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

At least 19 recordsLinked to original sources

arXiv2026

An analysis of four stellar rings

About 50 years ago, one thousand ring-like structures (called stellar-rings) were discovered by Isserstedt (1968). They were believed to be groups of young stars formed by shell-like triggered star formation, which would make them excellent tracers of spiral arms, for example. Neglected for 40 years, we used highly accurate kinematic, astrometric, and photometric data to investigate the four most prominent stellar rings. The aim is to investigate if those structures are indeed physically related groups of stars. We used proper motions and parallaxes from the Gaia DR2 to calculate distances and to search for common properties. Color-magnitude diagrams using BVJHKs measurements were investigated and isochrones fitted. None of the four stellar rings consists of a physically related group of young stars. The location of stars in the line-of-sight mimics a ring-like structure on the sky. The color-magnitude diagrams are typical for an integrated field population and not for a young star cluster, for example. The currently available data are sufficient to analyze ring-like structures with a high statistical significance. This allows a new search for such structures in the Milky Way.

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arXiv2024

Apparent non-variable stars from the Kepler mission

The analysis of non-variable stars is generally neglected in the literature. However, such objects are needed for many calibration processes and for testing pulsational models. The photometric time series of the Kepler satellite mission still stand as the most accurate data available today and are excellently suited to the search for non-variable stars. We analysed all long-cadence light curves for stars not reported as a variable so far from the Kepler satellite mission. Using the known characteristics and flaws of these data sets, we defined three different frequency ranges where we searched for non-variability. We used the Lomb-Scargle periodogram and the false-alarm probability (FAP) to analyse the cleaned data sets of 138 451 light curves. We then used log FAP > -2 to define a star as "non-variable" in the ranges below 0.1 c/d, 0.1 to 2.0 c/d, and 2.0 to 25.0 c/d, respectively. Furthermore, we also calculated the standard deviation of the mean light curve to obtain another parameter. In total, we found 14 154 stars that fulfil the set

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arXiv2021

A Kepler K2 view of subdwarf A-type stars

The spectroscopic class of subdwarf A-type (sdA) stars has come into focus in recent years because of their possible link to extremely low-mass white dwarfs, a rare class of objects resulting from binary evolution. Although most sdA stars are consistent with metal-poor halo main-sequence stars, the formation and evolution of a fraction of these stars are still matters of debate. The identification of photometric variability can help to put further constraints on the evolutionary status of sdA stars, in particular through the analysis of pulsations. Moreover, the binary ratio, which can be deduced from eclipsing binaries and ellipsoidal variables, is important as input for stellar models. In order to search for variability due to either binarity or pulsations in objects of the spectroscopic sdA class, we have extracted all available high precision light curves from the Kepler K2 mission. We have performed a thorough time series analysis on all available light curves, employing three different methods. Frequencies with a signal-to-noise ratio higher than four have been used for further analysis. From the 25 targets, 13 turned out to be variables of different kinds (i.e. classical pulsating stars, ellipsoidal and cataclysmic variables, eclipsing binaries, and rotationally induced variables). For the remaining 12 objects, a variability threshold was determined.

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arXiv2014

The first Delta a observations of three globular clusters

Globular clusters are main astrophysical laboratories to test and modify evolutionary models. Thought to be rather homogeneous in their local elemental Distribution of members, results suggest a wide variety of chemical peculiarities. Besides different main sequences, believed to be caused by different helium abundances, peculiarities of blue horizontal-branch stars and on the red giant branch were found. This whole zoo of peculiar objects has to be explained in the context of stellar formation and evolution. The tool of Delta a photometry is employed in order to detect peculiar stars in the whole spectral range. This three filter narrow band system measures the flux distribution in the region from 4900 to 5600A in order to find any peculiarities around 5200A. It is highly efficient to detect classical chemically peculiar stars of the upper main sequence, Be/Ae, shell and metal-weak objects in the Milky Way and Magellanic Clouds. We present Delta a photometry of 2266 stars from 109 individual frames for three globular Clusters (NGC 104, NGC 6205, and NGC 7099). A comparison with published abundances, for three horizontal-branch stars, only, yield an excellent agreement. According to the 3 sigma detection limit of each globular cluster, about 3% of the stars lie in abnormal regions in the diagnostic diagrams. The first observations of three widely different aggregates give very promising results, which will serve as a solid basis for follow-up observations including photometric as well as spectroscopic studies.

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arXiv2014

Photoelectric search for peculiar stars in open clusters. XV. Feinstein 1, NGC 2168, NGC 2323, NGC 2437, NGC 2547, NGC 4103, NGC 6025, NGC 6633, Stock 2, and Trumpler 2

The chemically peculiar (CP) stars of the upper main sequence are mainly characterized by strong overabundances of heavy elements. Two subgroups (CP2 and CP4) have strong local magnetic fields which make them interesting targets for astrophysical studies. This star group, in general, is often used for the analysis of stellar formation and evolution in the context of diffusion as well as meridional circulation. In continuation of a long term study of CP stars (initiated in the 1980ies), we present new results based on photoelectric measurements for ten open clusters that are, with one exception, younger than 235Myr. Observations in star clusters are favourable because they represent samples of stars of constant age and homogeneous chemical composition. The very efficient tool of Delta a photometry was applied. It samples the flux depression at 5200A typically for CP stars. In addition, it is able to trace emission line Be/Ae and lambda Bootis stars. Virtually all CP2 and CP4 stars can be detected via this tool. For all targets in the cluster areas, we performed a kinematic membership analysis. We obtained new photoelectric Delta a photometry of 304 stars from which 207 objects have a membership probability higher than 50%. Our search for chemically peculiar objects results in fifteen detections. The stars have masses between 1.7 M(Sun) and 7.7 M(Sun) and are between the zero- and terminal-age-main-sequence. We discuss the published spectral classifications in the light of our Delta a photometry and identify several misclassified CP stars. We are also able to establish and support the nature of known bona fide CP candidates. The new and confirmed CP stars are interesting targets for spectroscopic follow-up observations to put constraints on the formation and evolution of CP stars.

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arXiv2014

Spectrophotometric analysis of the 5200A region for peculiar and normal stars

Many chemically peculiar (CP) stars, especially the magnetic CP2 stars, show a flux depression at 5200A. The "Delta a" photometric System takes advantage of this characteristic to detect these objects in an efficient way. In addition, it is capable of finding metal-weak, emission-type, and shell-type objects of the upper main sequence. To compare available observations and to detect new peculiar objects, we used a spectrophotometric catalogue consisting of 1159 stars. From this catalogue, we selected 1067 objects to synthesize three different "a" indices to find the most efficient one for further observations. In addition, we extended the analysis to stars cooler than F5. We employed classical "Delta a" photometry described by Maitzen, using simulated filter curves, the spectrophotometric "Delta a" index by Adelman, and a modified index. During this investigation magnetic chemically peculiar stars could be confirmed using the data of the catalogue and in addition we noticed an unknown behavior for red stars in "Delta a", becoming independent of B-V. Even though the accuracy of the spectrophotometry used for this investigation is significantly lower than the photometric "Delta a" measurements, we are able to confirm peculiarity for most of the known CP2 stars above a certain limit of "Delta a". We investigated 631 stars hotter than spectral type F5 to find additional that are not yet identified peculiar objects. We find that for very low mass stars (M0), the "a" index is independent of the colour (effective temperature). The "Delta a" photometric system is very closely correlated with the effective temperature over a wide range of the main sequence. It is able to detect any kind of peculiarity connected to the 5200A region. Especially for low-mass stars, this opens up a new possibility of detecting peculiar objects in an efficient way.

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arXiv2010

A photometric long-term study of CP stars in open clusters

Photometric variability of chemically peculiar (CP) stars of the upper main sequence is closely connected to their local stellar magnetic field and their rotational period. Long term investigations, as presented here, help us to identify possible stellar cycles (as in the Sun). Furthermore, these data provide a basis for detailed surface mapping techniques. Photoelectric Stroemgren uvby time series for 27 CP stars within the boundaries of open clusters are presented. In addition, Hipparcos photometric data (from 1989 to 1993) are used for our analysis. Our observations cover a time period of about six years (1986 to 1992) with typically fifteen measurements for each objects. These observations help us to determine the rotational periods of these objects. A standard reduction procedure was applied to the data. When possible, we merged our data sets with already published ones to obtain a more significant result. A detailed time series analysis was performed, involving five different methods to minimize spurious detections. We established, for the first time, variability for fourteen CP stars. For additional two stars, a merging of already published data sets, resulted in more precise periods, whereas for six objects, the published periods could be confirmed. Last, but not least, no significant variations were found for five stars. Apart from six stars, all targets seem to be members of their host open clusters.

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arXiv2008

Chemically peculiar stars and their temperature calibration

The determination of effective temperature for chemically peculiar (CP) stars by means of photometry is a sophisticated task due to their abnormal colours. Standard calibrations for normal stars lead to erroneous results and, in most cases corrections are necessary. In order to specify appropriate corrections, direct temperature determinations for 176 objects of the different subgroups were collected from the literature. This much larger sample than in previous studies therefore allows a more accurate investigation, mostly based on average temperatures. For the three main photometric systems (UBV, Geneva, Stromgren uvbybeta), methods to determine effective temperature are presented together with a comparison with former results. Based on the compiled data we provide evidence that He (CP4) objects also need a considerable correction, not noticed in former investigations due to their small number. Additionally, a new relation for the bolometric correction and the capability of standard calibrations to deduce interstellar reddening for magnetic CP stars are shown.

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arXiv2006

CCD photometric search for peculiar stars in open clusters. VIII. King 21, NGC 3293, NGC 5999, NGC 6802, NGC 6830, Ruprecht 44, Ruprecht 115, and Ruprecht 120

We continue our survey for magnetic chemically peculiar (CP2) stars in galactic open clusters to shed more light on their origin and evolution. To study the group of CP2 stars, it is essential to find these objects in different galactic environments and at a wide range of evolutionary stages. The knowledge of open clusters ages and metallicities can help to find a correlation of these parameters with the (non-)presence of peculiarities which has to be taken into account in stellar evolution models. The intermediate band Delta a photometric system samples the depth of the 5200A flux depression by comparing the flux at the center with the adjacent regions with bandwidths of 110A to 230A. It is capable to detect magnetic CP2 and CP4 stars with high efficiency, but also the groups of (metal-weak) lambda Bootis, as well as classical Be/shell stars can be successfully investigated. In addition, it allows to determine the age, reddening and distance modulus with an appropriate accuracy by fitting isochrones. From the 1677 observed members of the eight open clusters, twenty five CP2 and one Ae stars were identified. Further nineteen deviating stars are designated as questionable due to several reasons. The estimated age, reddening and distance for the programme clusters were compared with published vales of the literature and discussed in this context. The current paper shows that CP2 stars are present continuously in very young (7Myr) to intermediate age (500Myr) open clusters at distances larger than 2kpc from the Sun.

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arXiv2006

Chemically peculiar stars in the Large Magellanic Cloud

The detection of magnetic chemically peculiar (CP2) stars in open clusters of extragalactic systems can give observational answers to many unsolved questions. The mean percentage of CP2 stars in the Milky Way is of the order of 5% for the spectral range from early B- to F-type, luminosity class V objects. The origin of the CP2 phenomenon seems to be closely connected to the overall metallicity and global magnetic field environment. The theoretical models are still only tested by observations in the Milky Way. It is therefore essential to provide high quality observations in rather different global environments. The young clusters NGC 2136/7 were observed in the Delta a photometric system. This intermediate band photometric system samples the depth of the 520nm flux depression by comparing the flux at the center with the adjacent regions with bandwidths of 11nm to 23nm. The Delta a photometric system is most suitable for detecting CP2 stars with high efficiency, but is also capable of detecting a small percentage of non-magnetic CP objects. We present high precision photometric Delta a observations of 417 objects in NGC 2136/7 and its surrounding field, of which five turned out to be bona fide magnetic CP stars. In addition, we discovered two Be/Ae stars. From our investigations of NGC 1711, NGC 1866, NGC 2136/7, their surroundings, and one independent field of the LMC population, we derive an occurrence of classical chemically peculiar stars of 2.2(6)% in the LMC, which is only half the value found in the Milky Way. The mass and age distribution of the photometrically detected CP stars is not different from that of similar objects in galactic open clusters.

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arXiv2006

An empirical temperature calibration for the Delta a photometric system. II. The A-type and mid F-type star

With the Delta a photometric system, it is possible to study very distant galactic and even extragalactic clusters with a high level of accuracy. This can be done with a classical color-magnitude diagram and appropriate isochrones. The new calibration presented in this paper is a powerful extension. For open clusters, the reddening is straightforward for an estimation via Isochrone fitting and is needed in order to calculate the reddening-free, temperature sensitive, index (g1-y)0. As a last step, the calibration can be applied to individual stars. Because no a-priori reddening-free photometric parameters are available for the investigated spectral range, we have applied the dereddening calibrations of the Stromgren uvbybeta system and compared them with extinction models for the Milky Way. As expected from the sample of bright stars, the extinction is negligible for almost all objects. As a next step, already established calibrations within the Stromgren uvbybeta, Geneva 7-color, and Johnson UBV systems were applied to a sample of 282 normal stars to derive a polynomial fit of the third degree for the averaged effective temperatures to the individual (g1-y)0 values with a mean of the error for the whole sample of Delta T(eff) is 134K, which is lower than the value in Paper I for hotter stars. No statistically significant effect of the rotational velocity on the precision of the calibration was found.

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arXiv2006

Photometric survey of marginally investigated open clusters: I. Basel 11b, King 14, Czernik 43

To progress in galactic studies based on open clusters, e.g. cluster formation rate or kinematical properties, one needs to improve the number of open clusters observed. However, only half of the 1700 known galactic open clusters have been properly observed so far, making any statistical investigation insignificant, especially at larger distances from the Sun. We study marginally investigated or neglected open clusters with Bessell CCD BVR photometry, whose data were used to fit isochrones to the individual color-magnitude diagrams. We examined the galactic clusters Basel 11b, King 14 and Czernik 43, the last being observed for the first time to this extent. As well as a careful comparison to available photometry, their parameters such as age, interstellar reddening, distance and apparent diameter were determined. The obtained cluster properties were verified by near infrared 2MASS data. The three investigated intermediate age clusters are all located in the galactic disk with distances between 1.8 and 3.0kpc from the Sun.

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arXiv2006

CCD photometric search for peculiar stars in open clusters. VII. Berkeley 11, Berkeley 94, Haffner 15, Lynga 1, NGC 6031, NGC 6405, NGC 6834 and Ruprecht 130

The detection of magnetic chemically peculiar (CP2) stars in open clusters of the Milky Way can be used to study the influence of different galactic environments on the (non-)presence of peculiarities, which has to be taken into account in stellar evolution models. Furthermore it is still unknown if the CP2 phenomenon evolves, i.e. does the strength of the peculiarity feature at 5200A, increase or decrease with age. We have observed eight young to intermediate age open clusters in the Delta a photometric system. This intermediate band photometric system samples the depth of the 5200A, flux depression by comparing the flux at the center with the adjacent regions having bandwidths of 110A, to 230A. The Delta a photometric system is most suitable to detect CP2 stars with high efficiency, but is also capable of detecting a small percentage of non-magnetic CP objects. Also, the groups of (metal-weak) lambda Bootis, as well as classical Be/shell stars, can be successfully investigated. This photometric system allows one to determine the age, reddening and distance modulus by fitting isochrones. Among the presented sample of eight galactic clusters, we have detected twenty three CP2, eight Be/Ae and eight metal-weak stars. Another six objects show a peculiar behaviour which is most probably due to a non-membership,variability or duplicity. Fitting isochrones to Delta a photometry yields estimates of the age, reddening and distance that are in excellent agreement with published values.

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arXiv2005

An empirical temperature calibration for the Delta a photometric system. I. The B-type stars

We establish an empirical effective temperature calibration of main sequence, luminosity class V to III B-type stars for the Delta a photometric system which was originally developed to detect magnetic chemically peculiar objects of the upper main sequence (early B-type to early F-type) at 5200A. However, this system provides the index (g1-y) which shows an excellent correlation with (B-V) as well as (b-y) and can be used as an indicator of the effective temperature. This is supplemented by a very accurate color-magnitude diagram, y or V versus (g1-y), which can be used, for example, to determine the reddening, distance and age of an open cluster. This makes the Delta a photometric system an excellent tool to investigate the Hertzsprung-Russell-Diagram (HRD) in more detail. Using the reddening-free parameters and already established calibrations within the Stromgren uvbybeta, Geneva 7-color and Johnson UBV systems, a polynomial fit of third degree for the averaged effective temperatures to the individual (g1-y)0 values was derived. For this purpose, data from the literature as well as new observations were taken resulting in 225 suitable bright normal B-type objects. The statistical mean of the error for this sample is 238K which is sufficient to investigate the HRD of distant galactic open clusters as well as extragalactic aggregates in the future.

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arXiv2005

CCD photometric search for peculiar stars in open clusters. VI. NGC 1502, NGC 3105, Stock 16, NGC 6268, NGC 7235 and NGC 7510

In a sample of six young open clusters we investigated 1753 objects using the narrow band, three filter Delta a photometric system resulting in the detection of eleven bona-fide magnetic chemically peculiar (CP) stars and five Be or metal-weak stars. The results for the distant cluster NGC 3105 is most important because of the still unknown influence of the global metallicity gradient of the Milky Way. These findings confirm that CP stars are present in open clusters of very young ages (log t > 6.90) at galactocentric distances up to 11.4 kpc. For all programme clusters the age, reddening, and distance modulus were derived using the corresponding isochrones. Some additional variable stars within Stock 16 could be identified by comparing different photometric studies.

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arXiv2005

CCD-Delta a and BVR photometry of NGC 7296

The first CCD photometric investigation of the open cluster NGC 7296 up to now was performed within the narrow band Delta a photometric system, which enables us to detect peculiar objects. A deeper investigation of that cluster followed, using the standard BVR-Bessel filter set. The age and E(B-V) was determined independently to log t= 8.0 and 0.15 respectively by using Delta a and broadband photometry. In total five Be/Ae objects and two metal-weak stars showing significant negative Delta a-values as well as one classical chemically peculiar star could be identified within that intermediate age open cluster.

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arXiv2005

On the incidence of chemically peculiar stars in the Large Magellanic cloud

With the aim to corroborate the result of a search for chemically peculiar stars in the Large Magellanic Cloud (LMC) we present measurements obtained from CCD-imaging of two fields, one containing a young open cluster (NGC 1711). While for the latter field, including its surrounding we obtain a contribution of three percent of chemically peculiar stars detectable by Delta a photometry (i.e. the magnetic objects of this group), the second field yields about half of this value in good accordance with the finding for NGC 1866 (Maitzen et al., 2001, A&A, 371, L5) the surrounding field of which has been found to exhibit a very low value of such stars - 0.3%. Thus we are faced with the fact, that our incipient impression about a substantially lower appearance of magnetic chemically peculiar stars in the LMC as compared to the Galaxy continues to be valid. Most of the photometrically identified peculiar stars (from their historical origin denominated Ap-stars) are located in the domain of the B-type stars. But this is a selection effect due to the limiting magnitude of our observing conditions impeding the observation of fainter main sequence stars. In addition to objects showing up as positive deviators in Delta a photometry we also discuss nine stars which appear opposite the main line of normal stars, hence are negative deviators. For most of them the interpretation as emission stars of B-type seems to be appropriate. The statistically relevant number of observations obtained so far in the LMC supports the view that the formation of magnetic peculiar stars has occurred there at a significantly lower rate.

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arXiv2005

On the detection of chemically peculiar stars using Delta a photometry

We have summarized all Delta a measurements for galactic field stars (1474 objects) from the literature published over more than two decades. These measurements were, for the first time, compiled and homogeneously analyzed. The Delta a intermediate band photometric system samples the depth of the 520nm flux depression by comparing the flux at the center with the adjacent regions with bandwidths of 11 to 23nm. Because it was slightly modified over the last three decades, we checked for systematic trends for the different measurements but found no correlations whatsoever. The Delta a photometric system is most suitable to detecting magnetic chemically peculiar (CP) stars with high efficiency, but is also capable of detecting a small percentage of non-magnetic CP objects. Furthermore, the groups of (metal-weak) lambda Bootis, as well as classical Be/shell stars, can be successfully investigated. In addition, we also analyzed the behaviour of supergiants (luminosity class I and II). On the basis of apparent normal type objects, the correlation of the three sigma significance limit and the percentage of positive detection for all groups was derived. We compared the capability of the Delta a photometric system with the Delta (V1 - G) and Z indices of the Geneva 7-color system to detect peculiar objects. Both photometric systems show the same efficiency for the detection of CP and lambda Bootis stars, while the indices in the Geneva system are even more efficient at detecting Be/shell objects. On the basis of this statistical analysis it is possible to derive the incidence of CP stars in galactic open cluster and extragalactic systems including the former unknown bias of undetected objects.

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