The information is prepared on the basis of data from the information-analytical system RSF, informative part is represented in the author's edition. All rights belong to the authors, the use or reprinting of materials is permitted only with the prior consent of the authors.

Project titleEarliest stages of evolution of the chemically peculiar stars: Ap/Bp phenomenon in star forming regions.

AffiliationInstitute of Solar-Terrestrial Physics of Siberian Branch of Russian Academy of Sciences,

Research area 02 - PHYSICS AND SPACE SCIENCES, 02-704 - Physics and evolution of stellar and interstellar medium

Keywordschemically peculiar stars, Ap/Bp stars; Herbig Ae/Be stars, star formation, clusters and associations PMS evolution, stellar atmospheres, chemical composition

Annotation
About 15-20% of the Main Sequence stars of early spectral types (~ B5 - F0) have abnormal spectra, which indicate an excess or a deficiency of certain chemical elements in their atmospheres. Among the various types of chemically peculiar stars, the subgroup of the so-called Ap/Bp stars is of particular interest. These stars demonstrate abundances of the elements of the iron peak and rare-earth metals exceeding by the several orders of magnitude those in the Solar atmosphere. Another key characteristic of Ap / Bp stars is the presence of large-scale magnetic fields with strengths up to tens of kGs. According to modern concepts, the anomalous Ap/Bp spectra of stars are caused by surface inhomogeneities of the chemical composition (“spots”) arising from the selective diffusion of atoms and ions under the action of radiation pressure. This process becomes possible in a highly stabilized atmosphere as a result of the combined action of several factors: (a) the absence of convection; (b) slow rotation (weak meridional circulation); (c) the presence of a strong magnetic field. Investigation of chemically peculiar stars is one of the most dynamically developing branches of modern astrophysics. Being a pronounced observational characteristic, the peculiarity of the Ap/Bp type is tightly connected with such fundamental processes in stars as the evolution of magnetic fields and the angular momentum loss. One of the most important tasks in this field is to establish a reliable time scale for the existence of the Ap/Bp phenomenon and its relationship with the characteristics of stars at different evolutionary stages. Numerous studies of Ap / Bp stars in clusters of different ages have revealed that they occupy the entire width of the Main Sequence (MS): from the Zero Age MS (ZAMS) to the Terminal Age MS (TAMS). At the same time, extrapolation of the frequency of Ap / Bp stars as a function of age allows us to expect that the first signs of chemical peculiarity may appear even before their arrival at the MS (on Pre-main sequence, PMS phase). The search and investigation of very young Ap/Bp stars are of exceptional importance both for defining the characteristic time scale in diffusion theory and for understanding aspects of the early evolution of magnetic fields, the loss of angular momentum, and the combined role of these effects in accretion processes in PMS stars of intermediate masses. Young progenitors of MS stars of spectral types A and B are the Herbig Ae/Be stars, which are found in starforming regions, young clusters, and OB-associations. Objects of this type are the stars of intermediate masses (~ 2-10Msolar) located on the PMS evolutionary tracks. They actively accrete matter from their circumstellar disks, which are the site of ongoing formation of planetary systems. A survey of the magnetic fields Herbig Ae/Be stars, carried out over the past decade, revealed their frequency on the order of 5-10%, i.e. approximately consistent with the occurrence of magnetic Ap/Bp stars in the Main Sequence. However, the strengths of the magnetic fields in Herbig Ae/Be stars are much weaker and do not exceed several hundred Gs, in contrast to the typical kGs fields in the MS Ap/Bp stars. Despite targeted searches, only a few examples of Herbig stars with Ap/Bp chemical peculiarities are known to date. There are two possible alternatives to explain this issue: 1) Surface inhomogeneties of the chemical composition (and hence the Ap/Bp spectral pattern) are not formed at the PMS phase, but only upon reaching the ZAMS when accretion decays and in the relaxation of the local magnetized volumes formed at the Hayashi phase completes and leads to an increase of the global magnetic field. 2) Actually, the problem is in the limitations of the existed sample. The searches are carried out mainly among the Herbig Ae/Be stars; however, this may lead to an missing of the Ap/Bp stars which are also at the PMS phase, but does not possess prominent accretion signspots. The reason may be, for example, early dissipation of the accretion disk. From the abovementioned, it is obvious that extensive efforts are needed to expand the sample and studies of the Ap/Bp stars at the PMS phase of evolution and near ZAMS. That is why this subject is in the focus of attention of a number of leading international groups. The proposed project is also aimed at solving this problem. The most effective approach is to study candidates for young Ap/Bp stars in star forming regions and young clusters, since the determination of the group age is possible more accurately than for field objects. The scientific novelty of the project consists in the study of candidates for PMS Ap/Bp stars for which there are no determinations of fundamental parameters and chemical composition and which are located in the regions of current star formation with the ages of 0.6-1.5 million years. To solve this problem, it is proposed to combine modern methods of quantitative analysis of high-resolution spectra with a comprehensive study of young stars in their environment, which is necessary for determining the age by comparing the position of an ensemble of stars on color-magnitude diagrams (CMD) with theoretical isochrones. As a result, the parameters of the atmospheres, chemical composition, as well as the age and evolutionary status of the stars under study will be determined. Regardless of whether they located on the PMS tracks or at the beginning of their life on the MS, important information will be obtained about the time scale and the character of the Ap/Bp peculiarity development in stars of intermediate masses. Also, new data will be obtained on the stellar population, extinction and age gradient in their parental regions of star formation.

Expected results
After completion of the project, the following results will be obtained: 1) High-resolution spectra of candidates for young Ap / Bp stars located in regions of current star formation will be studied. By modeling these spectra, an accurate determination of their fundamental parameters will be performed: effective temperature, surface gravity, micro- and macroturbulent velocities in the atmosphere. Analysis of the abundances of chemical elements in their atmospheres will make it possible to quantitatively characterize their chemical peculiarity. These results will make it possible to add these data to international databases of chemically peculiar stars with known parameters and to determine the prospects of these objects for further searches for their magnetic fields. 2) The stars under investigation will be considered in the context of their parental regions of star formation. The usage of the surrounding young stellar population in the construction of CMD diagrams will make it possible to more accurately determine the group age, and will also bring new data on the extinction and age of stars in the considered regions of star formation. The new discoveries of PMS stars becomes possible as result of expanding a sample of objects in considering regions and carrying out additional observations. 3) A detailed discussion of the obtained results will be carried out in the context of the problem of underestimating Ap/Bp stars at the earliest stages of evolution/ As well the discussion of fundamental question of the time scale of Ap/Bp peculiarity formation in the atmospheres of stars of intermediate masses. Ways of further development of this field will be outlined. Thanks to the scientific relevance and importance of the project’s goal, the obtained results will be the world level research and also important for the further development of ideas about the early stages of the evolution of chemically peculiar stars and magnetic fields. Additional results will be obtained about young stellar population in the environment of target objects, which will expand our knowledge of the population and characteristics in selected star forming regions in the solar neighborhood. The immediate practical significance of the project results lies in the verification under astrophysical conditions and possible correction of the existing data on atomic transitions, which is important for a wide range of applied researches. Also, the results of the project will have methodological and educational significance, they could be included in the courses on modern achievements of astrophysics for students of astronomical departments of universities.

Annotation of the results obtained in 2022
During the second year of the project the investigation of the chemical composition of the young peculiar star BD+300549, located in the star-forming region NGC 1333, has been completed. Previously, our analysis revealed a significant (~0.8 dex) difference in the silicon abundance, determined by the lines of two ionization stages: Si II and Si III presented in its spectrum. This so called "Si II / III - anomaly" is observed in the spectra of a few other chemically peculiar as well as normal B type stars, but does not yet have a satisfactory explanation. In the framework of the study of the "Si II / Si III-anomaly" in the spectrum of BD+30 549 we performed non-LTE calculations for the silicon lines used in our analysis. Accounting for the nLTE effects made it possible to significantly reduce the difference in the abundances determined from Si II and Si III lines, but did not lead to its complete elimination. Additionally, other possibilities were investigated: the vertical stratification of silicon in the atmosphere of BD+30 549 and its non-uniform lateral distribution (spots). Consideration of each of these effects separately led to a better representation of the profiles of individual lines, but not to the disappearance of the abundance difference determined by the two ionization stages. We conclude that for the explanation of silicon anomalies in the atmospheres of early type stars it is necessary to consider both the nLTE effects and the three-dimensional distribution of the element in the region of line formation. This is a challenge for the development of methods and techniques of quantitative analysis of stellar spectra. Further study was focused on the young chemically peculiar star LkHa324/B, located in LDN 988 star-forming region. Atmospheric parameters were determined using the method of spectral synthesis. Examination of magnetosensitive lines in the spectrum of LkHa324/B indicated that it hosts large-scale magnetic field with ~3.5 kGs strength. Individual LTE abundances for 15 elements in the LkHa324/B atmosphere were determined. A deficit of helium and light elements C, O, Mg, Al was revealed. At the same time, silicon and iron peak elements showed significant excess in abundances with a maximum value of +2.8 dex (relative to the Sun) for chromium. The spectrum also possesses lines of rare-earth elements: praseodymium and neodymium in the second stage of ionization. The abundance of these elements also showed an excess up to +4 dex. A comparison of the abundances pattern of LkHa324/B with few young chemically peculiar Herbig Ae stars and Ap/Bp stars on the Main Sequence showed a good agreement with the latter group. This points to selective diffusion as the reason for the formation of the peculiar surface composition observed in LkHa324/B. The age and evolutionary status of LkHa324/B were examined along with the nearby stellar population in the LDN 988 star-forming region. The photometric and spectroscopic observations of stars in the western part of the cloud LDN 988, experiencing strong extinction and containing LkHa324/B were analyzed. It was found from kinematical characteristics and age together with LkHa324/B these stars constitute a uniform population with the young stars in the much better studied eastern part of the cloud. Thus, previously reported in the literature assumption, that the entire cloud LDN 988 is encompassed by active star formation was confirmed. A comparison of the position of LkHa324/B in the Hertzsprung-Ressel diagram with the theoretical evolutionary tracks and isochrones allowed us to determine the age of the star as ~2.9 Myr and establish that it has completed only ~78% of its PMS evolution. The existence of the circumstellar disk with an inner cavity, detected from the infrared excess, together with the bipolar outflow from its vicinity, implies that in the recently LkHa324/B evolved through the phase of accreting Herbig Ae star. Accretion and diffusion are competing processes in formation of the surface chemical composition. The enrichment of the surface layers by the gas with normal composition accreted from the disk leads to erasing the abundance pattern formed by diffusion. However, in the case of LkHa324/B, in spite of the recent termination of accretion activity, diffusion has already managed to form a pronounced Ap type abundances pattern. It can be assumed that the large-scale magnetic field of the star played a stabilizing role in these processes and favored efficient diffusion.

Publications

1. I.S. Potravnov, M.V. Eselevich, S.A. Artemenko, E.V. Klunko Исследования активности молодых звёзд в Саянской солнечной обсерватории ИСЗФ СО РАН "Россия и Монголия: результаты и перспективы научного сотрудничества", сборник трудов международной научной конференции. Иркутск, Издательство: Институт географии им. В.Б. Сочавы Сибирского отделения Российской академии наук, Сборник трудов международной научной конференции "Россия и Монголия: результаты и перспективы научного сотрудничества", стр.268-271 (year - 2022) https://doi.org/10.53954/9785604859506

2. Potravnov I., Mashonkina L., Ryabchikova T. BD +30°549: a young helium-weak silicon star in the NGC 1333 star-forming region Monthly Notices of the Royal Astronomical Society, Vol. 520, 1, pp.1296-1310 (year - 2023) https://doi.org/10.1093/mnras/stad193

3. Potravnov I., Ryabchikova T., Artemenko S., Eselevich M. Pre-Main Sequence Ap Star LkHα 324/B in LDN 988 Star Forming Region Universe, 9(5), 210 (year - 2023) https://doi.org/10.3390/universe9050210

Annotation of the results obtained in 2021
One of the still unresolved questions of the physics of intermediate-mass stars is: when chemically peculiar Ap/Bp stars develope their surface chemical composition? Does the young Pre main sequence chemically peculiar stars exist? The solution of this problem is tightly related with the improvement of understanding of the specifics of the evolution of the internal structure and magnetic fields of stars, the structure of their atmospheres and the diffusion processes therein. This project focuses on study two chemically peculiar stars, BD+30 549 and LkHa324B, located in young star-forming regions. The aim is to determine their atmospheric parameters, chemical composition and age in order to clarify their evolutionary status and possible reasons and mechanisms resulted in observed surface abundances anomalies. We examining their evolutionary status in the context of the characteristics of the young stellar population inhabiting their parent star-forming regions. In the first year of the project, post-processing and preparation for further analysis of high-resolution archival spectra of the studied stars BD+30 549 and LkHa324B were carried out. Lists of spectral lines used to determine atmospheric parameters and chemical composition were prepared. Based on the literature data, a sample of young stars - members of the NGC1333 cluster which also contains the star BD+30 549 was created. For the western part of the star-forming region L988, where LkHa324B is located, a selection of young stellar candidates was made with the astrometric criteria using the GAIA EDR3 data and 2MASS and WISE infrared data. Photometric and spectral observations of this region were carried out using the AZT-33IK telescope of the Sayan Solar Observatory of the ISTP SB RAS. Further work with this material will be carried out in the second year of the project. The atmospheric parameters of BD+30 549 were determined using the synthetic spectrum method, as well as the individual abundances of elements in its atmosphere. This made it possible to classify the star as B8p HewkSi. The characteristic features of BD+30 549 were the absence of detectable axial rotation and turbulent motions in the atmosphere. The prominent anomalies in the chemical composition were found: the helium underabundance of about two orders of magnitude relative to the Sun and one - two orders overabundance of silicon (measured by lines related to two different ionization stages). Traces of a vertical gradient in the abundances of different elements, which in turn is an indicator of selective diffusion in the stellar atmosphere, were also found and quantitatively studied. An analysis of the position of BD+30 549 on the Hertzsprung-Russell diagram, together with theoretical evolutionary tracks and isochrones, made it possible to determine a star mass of about 3.2 solar masses and an age of 2.5 million years, corresponding to the upper age limit of the low-mass population of the NGC1333 cluster. The star has not yet reached the Zero Age Main Sequence, although it is already close to it. Nevertheless, this means that the loss of angular momentum and the formation of the present surface chemical composition as a result of diffusion processes in the atmosphere occurred at Pre- main sequence stage.

Publications