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We present the results of the spectroscopic monitoring of the Herbig Ae star HD 179218 carried out at the Shamakha Astronomical Observatory from 2015 to 2021 and the $UBVRI$ photometry at the Crimean Astronomical Observatory, covering the period 2007–2021. This object has a unique orientation of its rotation axis close to ``pole-on". The H$\beta$ line profiles are a single emission profile overlapped by blue and red absorption features living about several days. The 17 dates of 77 the H$\beta$ and the He I $\lambda$5876 line profiles demonstrated the inverse P Cyg-type profile with the variable velocity of the red edge of the absorption component up to 400 km/s. The period of the variations $P = 1.341 \pm 0.002$ days determined by Lafler–Kinman method is in accordance with the expected rotation period of the star. This observational fact can be a signature of the magnetospheric character of accretion in HD 179218. The inclination angle of the rotation axis is estimated as $i = 23^{\circ} \pm 3^{\circ}$. The observed phenomena can be connected with a specificity of stellar magnetism of HD 179218. Additionally, it has been shown that equivalent widths of the H$\alpha$ and H$\beta$ lines as well as the photometric $UBVRI$ parameters of HD 179218 demonstrate cyclic variability with several periods falling in the range from 19 days to 737 days. A possible interpretation of these phenomena can be a result of presence near HD 179218 of a low-mass binary component or exoplanets. A sudden drop of circumstellar extinction took place in October of 2011, possible interpretations of this phenomenon are considered.
stars: variables: Herbig Ae/Be, circumstellar matter; accretion; individual: HD 179218
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\begin{document}
\title{Spectral and photometrical peculiarities of the unusual Herbig Ae star HD\,179218}
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\titlerunning{HERBIG AE STAR HD\,179218} % abbreviated title (for running head)
%also used for the TOC unless
%\toctitle is used
\author{N.Z.~Ismailov\inst{1} \and M.A.~Pogodin\inst{2} \and A.F.~Kholtygin\inst{3} \and H.N.~Adigozalzade\inst{1} \and U.Z.~Bashirova\inst{1} \and O.V.~Kozlova\inst{4}
\and A.A.~Ivanova\inst{3}}
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\authorrunning{Ismailov et al.} % abbreviated author list (for running head)
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%%%% list of authors for the TOC (use if author list has to be modified)
%\tocauthor{}
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\institute{Shamakha Astrophysical Observatory, Shamakha, Azerbaijan,\\
\email{ismailovnshao@gmail.com}, \and
Central (Pulkovo) Astronomical Observatory, Russian Academy of Sciences, St.Petersburg, Russia, \and
Saint-Petersburg University, Russia, \and
Crimean Astrophysical Observatory Russian Academy of Sciences, Naychny, Crimea, Russia}
\abstract{We present results of the spectroscopic monitoring of the Herbig Ae star HD\,179218 carried out at the Shamakha AO from 2015 to 2021 and the UBVRI photometry at the Crimean AO, covering
the 2007-2021 period. This object has unique a specific orientation of its rotation axis close to "pole-on". The H-beta line
profiles in the spectrum of HD\,179218 look as a single emission profile overlapped by local blue and red absorption features living about several days. The 17 dates of 77 the H-beta and the HeI\,5876 line profiles demonstrated the inverse P\,Cyg-type profile with the variable velocity of the red edge of the absorption component up to $400\,$km/s. The period of the variations $P = 1.341 \pm 0.002\,$days determined by Lafler-Kinman method is in accordance with the expected rotation period of the star. This observational fact can be a signature of the magnetospheric character of accretion in HD179218. The inclination angle of the rotation axis is estimated as
$i = 23 \pm 3^\circ$. The observed phenomena can be connected with a specificity of stellar magnetism of HD\,179218. Additionally, it has been shown that equivalent widths of the H$\alpha$ and H$\beta$ lines as well as the photometric UBVRI parameters of HD\,179218 demonstrate cyclic variability with several periods falling in the range from 19 to 2981 years. A possible interpretation of these phenomena can be a result of presence near HD\,179218 of a low-mass binary component or exoplanets. A sudden drop of circumstellar extinction took place in October of 2011, the possible interpretations of this phenomenon is considered.
\keywords{stars: Herbig AeBe stars: individual: HD\,179218; circumstellar envelope; accretion}
\doi{10.26119/VAK2024-ZZZZ}
}
\maketitle
%=======================================================================1.
\section{Introduction}
\label{s.Intro}
Herbig Ae/Be stars are conventionally regarded as pre-main sequence (PMS) objects of intermediate mass (2 to 10 $M_\odot$) \citep{Herbig1960,Finkenzeller-1984,The-1994}. They are
surrounded by circumstellar (CS) envelopes containing the equatorial accretion disk and a matter outflow (wind) zone at higher latitudes. The Herbig Ae star HD\,179218 demonstrates a "pole-on"
orientation relative to the line-of-sight. According to~\cite{Leinert-2004} its inclination angle $i$ is near $20^{\circ}$. We present here the results of our investigation of this object.
%=======================================================================2.
\section{Observations}
\label{s.Observations}
We carried out spectroscopic monitoring of HD\,179218 from 2015 to 2021 with echelle-spectrometer installed at the 2-m telescope of the Shamakha Astrophysical Observatory of the
Azerbaijan National Academy of Sciences. In all, 86 spectra have been obtained. Additionally, we used in our investigation the results of UBVRI photometry of the object, performed
at the Crimean Astrophysical Observatory of the Russian Academy of Sciences between 2007 and 2021 (76 observing dates).
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%=Fig_01
\begin{figure*}[ht!]
\centering
\includegraphics[scale=0.60]{Fig01_IsmailovEtal.pdf}
\caption{Normalized to the continuum level H$_\beta$ line profiles observed at different dates in the spectrum of HD\,179218 after subtracting the stellar atmospheric component:
a) single emission profiles; b) P\,Cyg-type profiles; c) inverse P\,Cyg-type profiles; d) multi-component profiles.
}
\label{Fig.HD179218_Hbeta}
\end{figure*}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%=Fig_01
%===============================================================3.
\section{Short-term spectral variability}
\label{s.ShortVary}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%=Fig_02
\begin{figure*}[ht!]
\centering
\includegraphics[scale=0.60]{Fig02_IsmailovEtal.pdf}
\caption{The the Lafler \& Kinman periodogram for $V_\mathrm{red}$ values for H$\beta$ line (top panel) and He\,I\,5876 (bottom panel). A zero time corresponds to MJD\,57000.0
}
\label{Fig.HD179218_LaflerKinman1965}
\end{figure*}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%=Fig_02
We used temporal behavior of the H$_\beta$ profile in spectra of HD\,179218 to investigate structural and kinematic properties of its CS gas in the region near the line-of-sight between the star and the observer. This object demonstrates a wide variety of H$_\beta$ profile types observed in different dates: simple single emission profiles, the P\,Cyg-type and the inverse P\,Cyg-type profiles, and, at last, multi-component emission profiles with blue as well as red absorption features as it seen in Fig.\ref{Fig.HD179218_Hbeta}. Among profiles with the inverse P\,Cyg-feature, the velocity of the red boundary of the absorption component varied from 235 to 390 km/s in different dates (17 dates). In these dates profiles of another emission line He\,I\,7876 also demonstrate the inverse P\,Cyg-type structure with the velocity of the red edge of the absorption component from $50$ to $450\,$km/s. Using the \cite{LaflerKinman1965} method we have found the period of the line profile variability for both lines
H$_\beta$ and HeI\,5876. The corresponding periodograms are presented in Fig.~\ref{Fig.HD179218_LaflerKinman1965}.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%=Fig_03
\begin{figure*}[ht!]
\centering
\includegraphics[scale=0.60]{Fig03_IsmailovEtalA.pdf}
\includegraphics[scale=0.72]{Fig03_IsmailovEtalB.pdf}
\caption{{\bf Left panel:} profiles of the H$_\beta$ line obtained on 29/30 July 2017 with interval in time 10 hours (a and b) and August 2, 2017 (c). The vertical dashed line marks the not displaced during 3 days absorption feature. {\bf Right panel:} the same as in the left panel, but for dates July 7 and 8, 2019. Two local absorption features at $-75$ and $+115\,$km\,s$^{-1}$ are seen.
}
\label{Fig.HbetaVary}
\end{figure*}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%=Fig_03
The mean value of the revealed period and its error is $P = 1.341 \pm 0.002\,$ days with the use of four independent estimations (P and doubled P for both lines seen in periodograms). This value is in correspondence with the expected rotation period of HD\,179218. This result can be estimated as a signature of magnetospheric accretion in HD\,178218. The values of the rotation period allows to estimate the inclination angle of the rotation axis $i = 23 \pm 3^\circ$, which is in a good accordance with~\cite{Leinert-2004}.
Besides of the periodic variations of the H$_\beta$ line profile, some other types of variability were registered in spectra of HD\,179218. The most commonly occurring one is an appearance of local blue or red local absorption features overlapping a single emission profile with lifetime of several days and stable velocity positions. Examples of such phenomenon are illustrated in Fig.~\ref{Fig.HbetaVary}. Such temporal behavior excludes rotational modulation of line profiles as a result of eclipsing the star by azimuthal CS inhomogeneities, since the rotation period of the star is only $1.34\,$days. We are forced to accept that at the present time we unable to propose a single certain interpretation of this phenomenon. We assume that it can be connected with a specificity of stellar
magnetism of HD\,179218.
%================================================4.
\section{Long-term photometric variability}
\label{s.LongVary}
To analyze the variability we used photometric data obtained in five bands of the UBVRI system in 2007–2021 at the AZT-11 telescope in Crimean Astrophysical Observatory with the 5-channel Piirola photometer-polarimeter.
Besides of the cyclic variability, a remarkable abrupt brightness jump was observed in October 2011 in all UBVRI photometric bands. The maximal rise in brightening was in U, then its magnitude decreased from U to V. It became almost unnoticed in the band R, and in the band I the brightness, in contrary, suddenly decreased. The UVI light curves, illustrating this phenomenon, are shown in Fig.~\ref{Fig.UBVRI}~(left panel). Dependencies between stellar magnitudes in the UVI bands on V are given in Fig.~\ref{Fig.UBVRI}~(right panel). We assume that the phenomenon can be connected with a sudden disappearance of dust from the CS region between the star and the observer. This has to lead to a reduce of CS extinction and, consequently, to a rise of brightness in the visual spectral range. But such reduce of amount of CS dust, responsible for the near-IR excess, can result in a fall emission in the band I, where this excess starts to be observed.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%=Fig_04
\begin{figure*}[ht!]
\centering
\includegraphics[scale=0.56]{Fig04_IsmailovEtala.pdf}
\includegraphics[scale=0.38]{Fig04_IsmailovEtalb.pdf}
% \includegraphics[scale=0.36]{Photometry.pdf}
\caption{{\bf Left panel:} time dependencies of the stellar magnitudes of HD\,179218 in the bands U, V, and I before and after the jump time $T_j$.
{\bf Right panel:} dependence U, V, and I magnitudes on V. }
\label{Fig.UBVRI}
\end{figure*}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%=Fig_04
Also, we can speculate, that all active phenomena, observed in HD\,179218 at the time scales of order of years can be connected with: a) existence in the CS disk of dense local bodies (protoplanets), rotating at different distances from the star; b) formation in the disk of azimuthally inhomogeneous (spiral-like) structures \citep{Boccaletti-2020}, and c) presence of small-mass components of the main A0e object.
%======================================================5.
\section{CONCLUSIONS}
\label{s.Concl}
In the present paper we continue the study of a very rare subgroup of young Herbig Ae/Be stars: class A stars with an orientation of the rotation axis relative to the
observer close to pole. It is from this angle one can observe the circumpolar regions of the circumstellar envelope, where the gas has the least studied structure and kinematic.
A cyclic variability of the red boundary of the absorption component $V_{\mathrm{red}}$ of the H$\beta$ and HeI\,5876 line profiles with a period $P = 1.34\,$days corresponding to the expected rotation period of the star, was discovered. The maximum values of the parameter $V_{\mathrm{red}}$ reach $350-400\,$km/s, which is possible only with the magnetospheric nature of accretion. The angle of inclination of the star's rotation axis to the line of sight is estimated to be $i = 23 \pm 3^\circ$.
The analysis of the H$_\alpha$ and H$_\beta$ line profiles in the HD\,179218 spectra and photometric data in the UBVRI bands carried out in this work showed the presence of regular spectral and photometric variability with long periods. Such variability may be associated with changes in the accretion rate and precession of the accretion disk. Regular variability over long periods of time may indicate the presence of a distant satellite and, possibly, a distant exoplanet/exoplanets.
Analysis of the dependencies of the stellar brightness in the I band and the V-I color index on the magnitude in the V filter indicates an increase in the absorption of the star's radiation in the I band in the period from September 20, 2011 to October 3, 2011. This increase in absorption continued through 2021.
%%% bibliography
\bibliographystyle{aa}
\bibliography{Ismailov}
\end{document}
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3. Herbig G.H., 1960, Astrophysical Journal Supplement, 4, p. 337
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5. Leinert C., van Boekel R., Waters L.B.F.M., et al. 2004, Astronomy \& Astrophysics, 423, p. 537 %
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