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\input ibvs3.sty

\begin{document}
\IBVSheadDOI{63}{6254}{14 September 2018}

\IBVStitle{The variable carbon star CGCS 6107}


\IBVSauth{Nesci, R.$^1$; Calabresi, M.$^2$; Rossi, C.$^3$; Ochner, P.$^4$}

\IBVSinst{INAF/IAPS, via Fosso del Cavaliere 100, 00133 Roma, Italy, e-mail: roberto.nesci@iaps.inaf.it}
\IBVSinst{Frasso Sabino Observatory, MPC 157, Italy}
\IBVSinst{INAF/OAR, Monteporzio, Italy}
\IBVSinst{INAF/OAPD Asiago, and Universit\'a di Padova, Italy}

\SIMBADobj{CGCS 6107}

\IBVStyp{ SR }
\IBVSkey{photometry}
\IBVSabs{The spectroscopic and photometric variability of CGCS 6107 has been studied with four telescopes from 2015 to 2018. The star varied between R=11.4 and 14.2 mag with a time scale  of $\sim$500 days. An appreciable color variation was observed, the star being bluer when brighter. H$\alpha$ emission was present around maxima. The spectrum is that of an N type giant veiled by a variable dusty envelope. }


\begintext

\section {Introduction}
Carbon stars on the Asymptotic Giant Branch (AGB) are supposed to be in the last phase of stellar evolution after the Third Dredge-Up and before the ejection of the planetary nebula. Given their evolutionary status they are expected to be variable. 

Out of the 6891 stars listed in the Catalog of Galactic Carbon Stars (CGCS, Alksnis et al.\ 2001), 851 are reported as variables in the General Catalog of Variable stars (GCVS, Samus et al.\ 2017, CDS B/gcvs):  385 of them have also a period or variability time-scale reported, but only 150 are classified as Miras. The VSX catalog (Watson et al.\ 2016), updated more frequently, reports much more (1985) variables among the CGCS stars, 957 of them with a quoted period, but only 270 are classified as Miras or likely Miras: it appears therefore that only a minority of the AGB carbon stars are Mira variables.

Automatic surveys with robotic telescopes, dedicated to the detection of transient sources (Supernovae, Gamma Ray Bursts, Near Earth Asteroids, etc.) in large sky areas, contain large amounts of data which can significantly improve our knowledge in this topic: given that this is not the main goal of the science teams operating these telescopes, these large databases are still partially unexplored from this aspect.

We report here  the results of our recent study on the variability of the carbon star CGCS 6107, to stimulate  the curiosity for strongly variable sources  and  prompt similar  researches in the available databases.

\section{CGCS 6107}

The star (05:49:32.31 +46:35:57.9, J2000) is a very bright infrared source detected by IRAS, at low  galactic latitude in the Auriga constellation ($b=9.68$\deg). Its IRAS-LRS spectrum is classified F (Kwok et al.\ 1997) suggestive of a late spectral type M or C with small amount of circumstellar dust.

It was spectroscopically observed in the optical by Cohen et al.\ (1996) and classified as C-,4, with a significant H$\alpha$ emission. It is listed in the CGCS but without any indication of variability. It is not covered by the Sloan DR14\footnote{http://www.sdss.org/dr14/}.

The star is present in the main infrared catalogs: 2MASS (Cutri et al., 2003), WISE, (Cutri et al.\ 2013), and AKARI (Ishihara et al.\ 2010). The 2MASS $J-H$;$H-K$ colors of the star are $J-H=2.10$, $H-K=1.67$ mag, so it is located well inside the region of the moderately obscured carbon stars, even when small color changes are taken into account, but is not included in the catalog of Infrared Carbon Stars by Chen and Yang (2012).

Only in 2015 the star was pointed out as variable by the Japanese amateur astronomer Shigehisa Fujikawa (2015): spectra taken 3 days after discovery by Munari (2015) with the 122\,cm telescope of the Asiago (Pennar) Observatory showed a carbon star spectrum and confirmed  the presence of H$\alpha$ emission.

At the time of writing, the star is listed as variable in the VSX catalog but the variability amplitude is simply given by an upper limit.

\section{Photometric observations and calibrations}
Soon after Fujikawa's announcement, we started a photometric monitoring of CGCS 6107 with 3 telescopes: the 152\,cm of Loiano (Bologna Observatory), the 37\,cm of Frasso Sabino (IAU 157) and the 30 cm of Foligno Observatory (IAU K56). 
The Loiano and Frasso Sabino telescopes were equipped with CCD cameras and Bessell \textit{BVRI} filters; the Foligno telescope was equipped with a commercial digital camera  (DSLR, Nikon D90 up to 2018 and red extended Canon 550D camera afterwards).
Loiano and Frasso Sabino provided  good quality photometry in a few nights, Foligno allowed a denser monitoring with lower accuracy.

Twenty stars included in the field of view of all the involved telescopes were selected from the UCAC4 catalog to define a comparison sequence, and are listed in Table~1. Aperture photometry was performed using IRAF/apphot\footnote{IRAF is distributed by the NOAO, which is operated by AURA, under contract with NSF.} with radius equal to the average FWHM of each image.
 
The UCAC4 catalog gives magnitudes in the r$_{\rm Sloan}$ and i$_{\rm Sloan}$ bands, which are somewhat different from the Bessell's ones, and our star is quite red ($R-I \sim 2$), therefore a systematic color term is expected: however there were no stars of comparable colors in the field of view so that we could not compute reliable corrections. 
We feel this is not critical for the aim of this paper, devoted just to the study of the light curve and of possible color changes of the star, and not to a comparison with theoretical stellar atmosphere models.
A linear fit between instrumental and catalog magnitudes provided the calibration curve to evaluate the magnitude of the variable. The slope of the line was always very close to 1.0, as expected for an ideal linear detector. The rms deviation of the comparison stars magnitudes with respect to the fitting line was adopted as true photometric uncertainty of the variable star magnitude.
Given the non-standard color separation provided by the DSLR cameras of Foligno, we performed a few nearly simultaneous observations with the Frasso Sabino telescope to establish proper systematic corrections for the $V$ and $R$ bands.

The $V$ and r$_{\rm Sloan}$ magnitudes of our star were always inside, or shortly outside, the range of the comparison stars, while the i$_{\rm Sloan}$ magnitudes were always well outside the range, so these values are extrapolated and less reliable. 

\begin{table}
\caption{Comparison sequence for CGCS 6107.}
\begin{center}
\begin{tabular}{r r r r r}
\hline
RAJ2000&DJ2000&V&r$_{\rm Sloan}$&i$_{\rm Sloan}$\\
\hline
87.2972&+46.6587&15.011&14.675&14.325\\
87.3021&+46.5742&16.480&16.065&15.824\\
87.3159&+46.6300&14.974&14.666&14.386\\
87.3199&+46.6041&15.571&15.251&14.995\\
87.3203&+46.5450&14.776&14.306&13.855\\
87.3346&+46.6278&16.649&16.092&15.913\\
87.3428&+46.6124&16.144&16.008&15.855\\
87.3438&+46.6657&14.768&14.239&13.719\\
87.3502&+46.5741&14.503&14.205&13.931\\
87.3587&+46.6625&14.290&14.061&13.807\\
87.3587&+46.5612&15.088&14.811&14.559\\
87.3836&+46.5586&14.607&14.347&14.060\\
87.3851&+46.6097&15.209&14.835&14.455\\
87.3912&+46.6528&14.204&13.992&13.804\\
87.3993&+46.6128&14.210&13.943&13.652\\
87.4040&+46.6005&15.809&15.401&15.077\\
87.4108&+46.6139&15.749&15.439&15.092\\
87.4122&+46.6229&15.739&15.459&15.180\\
87.4423&+46.5407&15.542&15.031&14.431\\
87.4811&+46.6385&15.392&14.713&14.036\\
\hline
\end{tabular}
\end{center}
\end{table}

\section{Light curve}

Our photometric data in the $V$ and r$_{\rm Sloan}$ bands are reported in Table 2: column 1 is JD$-$2,400,000, columns 2, 3, 4 and 5 are  magnitudes with their errors, column 6 is the instrument used, coded as follows (FR= Frasso Sabino; EK= Cima Ekar; LO= Loiano; NI= Foligno with Nikon D90; CA= Foligno with Canon 550D).
Magnitudes fainter than $V \sim 16$ could not be measured with the 30cm telescope.

\begin{table}
\caption{Observed magnitudes of CGCS 6107 (all telescopes).}
\begin{center}
\begin{tabular}{l l l l l c}
\hline
JD&V&err\_V&r&err\_r&tel\\
\hline
57332&13.54&0.02&11.30&0.02&FR\\
57367&13.98&0.06&11.70&0.10&NI\\
57402&14.12&0.06&11.60&0.04&LO\\
57439&14.56&0.05&12.40&0.20&NI\\
57449&14.69&0.08&12.50&0.10&NI\\
57473&15.04&0.06&12.67&0.10&NI\\
57482&15.12&0.08&12.98&0.10&NI\\
57492&15.18&0.08&12.80&0.10&NI\\
57498&15.40&0.10&13.20&0.10&NI\\
57503&15.40&0.05&13.10&0.10&NI\\
57507&15.51&0.12&13.20&0.10&NI\\
57694&15.44&0.04&13.24&0.10&NI\\
57708&15.35&0.08&13.15&0.10&NI\\
57735&15.04&0.07&12.85&0.10&NI\\
57741&15.00&0.07&12.80&0.10&NI\\
57768&14.89&0.06&12.70&0.05&NI\\
57774&14.79&0.04&12.67&0.06&NI\\
57796&14.73&0.04&12.62&0.10&NI\\
57799&14.73&0.03&12.81&0.09&NI\\
57799&14.78&0.04&12.40&0.06&LO\\
57814&14.75&0.04&12.68&0.05&NI\\
57829&14.70&0.04&12.59&0.08&NI\\
57840&14.75&0.02&12.47&0.02&FO\\
57857&14.63&0.06&12.51&0.10&NI\\
57879&14.70&0.10&12.71&0.07&NI\\
58085&-----&-----&14.21&0.07&NI\\
58093&-----&-----&14.22&0.10&NI\\
58106&-----&-----&14.03&0.10&NI\\
58109&17.05&0.15&14.03&0.04&FR\\
58120&-----&-----&13.90&0.10&NI\\
58139&-----&-----&13.70&0.10&NI\\
58141&16.57&0.03&13.70&0.03&FR\\
58153&-----&-----&13.48&0.08&NI\\
58159&16.32&0.02&13.50&0.03&FR\\
58164&-----&-----&13.41&0.09&CA\\
58186&-----&-----&13.38&0.09&CA\\
58200&-----&-----&13.26&0.08&CA\\
58202&-----&-----&13.17&0.07&CA\\
58212&15.90&0.10&13.19&0.06&CA\\
58215&15.89&0.04&13.18&0.02&FR\\
58224&15.70&0.10&13.01&0.06&CA\\
58229&15.75&0.03&12.86&0.04&LO\\
58232&15.70&0.10&12.86&0.07&CA\\
\hline
\end{tabular}
\end{center}
\end{table}

 A light curve of our star starting  from 2014-01-19 can be recovered from the ASAS-SN database (Shappee et al.\ (2014); Kochanek et al.\ (2017)\footnote{\tt https://asas-sn.osu.edu}, which became public only in 2018.
These data are taken with an unfiltered FLI CCD camera and tied to Johnson's $V$-band using the APASS 9 catalog. Below $V\sim 16$, these $V$ magnitudes have uncertainties of several tenths, due to the short exposure times used by the survey.

Fig.~1 reports the ASAS-SN light curve (stars) and our r$_{\rm Sloan}$ light curve (squares) on a common magnitude scale, showing a very good agreement of the overall shape in the common time interval. 
The source is characterised by very large variations ($>2.5$ mag), with a time scale (peak to peak distance) of about 500 days: the variation amplitude is not constant, suggesting a classification of Semi Regular rather than of Mira type variability. As mentioned  in the Introduction, only a small fraction of the AGB carbon stars shows a regular Mira type light curve, so our finding is not unusual.  Similar large amplitude variations, superimposed on longer term trends in the light curve, have been reported also for other carbon stars with strong infrared excess recently studied by our group (see e.g. Gaudenzi et al.\ 2017; Nesci et al.\ 2018).


\IBVSfig{12cm}{6254-f1.eps}{The light curve of CGCS 6107 from our observations in the r band, (squares with error bars) and the ASAS-SN light curve in the V band (stars). Vertical scale in magnitudes. The letters in the upper side mark the dates of the spectroscopic observations listed in Table 5: E=Ekar, L=Loiano, P=Pennar.}
\IBVSfigKey{6254-f1.eps}{CGCS 6107}{light curve}

An FFT analysis with Period04 (Lenz and Breger 2005) of the ASAS-SN light curve  shows a main peak at 543 days, blended with its (fainter) alias at 1083 days; a further peak at 201 days has a low power and is of limited importance in the light curve fit. The period of 543 days is fully compatible with our dataset. 


\IBVSfig{12cm}{6254-f2.eps}{Phased optical light curve of CGCS 6107 from ASAS-SN data folded with the 543 day period. }
\IBVSfigKey{6254-f2.eps}{CGCS 6107}{light curve}

Despite that the star is a semiregular rather than a Mira, we show in Fig.~2 the optical light curve folded with the formal 543 days period. The substantial scatter around phase 0 is mainly due to the variable amplitude of the light curve, as apparent from Fig.~1.

Color indices ($V-r$) and ($r-i$) of the star were measured at different flux levels and are collected in Table 3: the star appears markedly redder when  fainter.
  
\begin{table}
\begin{center}
\caption{Color indices of CGCS 6107.}
\begin{tabular}{l c c c c}
\hline
Telescope        &           date &        r$_{\rm Sloan}$  &  $V-r_{\rm Sloan}$   & $r-i_{\rm Sloan}$ \\
\hline
Frasso Sabino & 2015-11-06 & 11.30 & 2.24 & 1.89 \\
Loiano              & 2016-01-15 & 11.61 & 2.50 & 1.95 \\
Loiano              & 2016-11-29 & 12.89 & 2.60 & 2.01 \\
Loiano              & 2017-02-14 & 12.40 & 2.38 & ---- \\
Frasso Sabino & 2017-03-27 & 12.47 & 2.28 & 2.10 \\
Frasso Sabino & 2017-12-22 & 14.04 & 3.14 & 2.44 \\
Frasso Sabino & 2018-01-23 & 13.70 & 2.86 & 2.37 \\
Frasso Sabino & 2018-02-10 & 13.50 & 2.82 & 2.31 \\
Frasso Sabino & 2018-04-06 & 13.18 & 2.71 & 2.24 \\
Loiano             & 2018-04-20 & 12.86 & 2.89 & 2.05 \\
\hline
\end{tabular}
\end{center}
\end{table}

We have also measured the star magnitudes on historic plates of the DSS, recoverable from the Space Telescope Science Institute, using our comparison sequence. The calibration curve was markedly non linear, so we could not measure the $B$ magnitude with our UCAC4 sequence because well outside the range. The results for the $V$, r$_{\rm Sloan}$, i$_{\rm Sloan}$ filters are collected in Table 4 and confirm the variability of the star in the past.


\begin{table}
\begin{center}
\caption{Observed magnitudes of CGCS 6107 (all telescopes).}
\begin{tabular}{l c c c  }
\hline
Emulsion & band & date & mag \\
\hline
103a-E & r &1952-12-21 & 12.6 \\
QuickV & V &1983-01-14 & 16.4 \\
IIIaF      & r &1989-10-05 &16.4\\
IIIaF      & r &1989-10-29 & 16.2 \\
IV-N      & i &1996-11-03 & 12.2\\
IV-N      & i &1999-10-13 & 12.3\\
\hline
\end{tabular}
\end{center}
\end{table}

\bigskip

In the infrared the star was observed for 3 years (from 1990-02-09 to 1993-04-15) with weekly sampling by the DIRBE instrument (Smith et al.\ 2004; Price et al.\ 2010) on board the COBE satellite,  in the 3.6 $\mu$m and 4.9 $\mu$m bands. The star was not classified as variable in the DIRBE catalog (Price et al.\ 2010) according to the strict criteria adopted, but  an eye inspection of the data suggested a possible variability. B.J. Smith kindly confirmed to us that no contamination by nearby sources was present for this star, so we made an independent analysis of the published data and we built the light curves at 3.5 and 4.9 $\mu$m applying a running mean of 5 consecutive measures: the result is shown in Fig.~3.

\IBVSfig{10cm}{6254-f3.eps}{The light curve of CGCS 6107 at 3.5 and 4.9 $\mu$m from the DIRBE data after a 5-point running mean. Error bars are the rms deviation form the mean of the averaged points. We remark that these errors are quite large and of very different size in different years. The 4.9 $\mu$m data seem of better quality.}
\IBVSfigKey{6254-f3.eps}{CGCS 6107}{light curve}

A peak-to-peak amplitude of about 1.3 mag is evident, with a time scale of about 500 days, similar to the optical one. The amplitude is similar to that measured for the `bona fide' variables of similar periods in the Price et al.\ (2010) catalog.   

A deeper analysis of the IR light curves in each band and of the averaged (3.5 and 4.9 $\mu$m) fluxes with the FFT technique shows several peaks in the power spectrum with comparable intensities and significantly different phases: for the averaged curve the peaks are around  558, 254, 133, 105, and 76 days (see Fig.4). The presence of so many peaks with similar power suggests a rather noisy pattern in the light curve: actually a single period is quite inadequate to reproduce its overall shape. The actual variability range and time scale are therefore ill-defined from these data. We recall that CGCS 6107 is near the detection limit of the DIRBE instrument, and some details of the light curve might be of instrumental origin. In the spectral energy distribution, the average DIRBE fluxes fit well between the 2MASS (1.25, 1.65, and 2.2 $\mu$m) and the AKARI (9 and 18 $\mu$m) values.

\IBVSfig{8cm}{6254-f4.eps}{The power spectrum of CGCS 6107 from the DIRBE data: it is evident that several frequencies of similar power are present, indicating a complex structure. }
\IBVSfigKey{6254-f4.eps}{CGCS 6107}{periodogram}


\section{Spectroscopic observations}
Spectra of the star were taken at different dates with the Asiago (Cima Ekar 182 cm and Pennar 122 cm) and the Loiano 152 cm Observatories,  with luminosity levels ranging from r = 11.4 to 13.7 mag; data reduction was performed with the standard IRAF  procedures. 
The observations log is given in Table 5: column 1 is the telescope, column 2 the date, column 3 the spectral resolution in \AA, column 4 the r magnitude at the time of observation, column 5 the H$\alpha$ equivalent width in \AA. These  last values are strongly affected by the variability of the continuum and typical errors are about  0.3 \AA .
In the last row we report the data relative to the observation by Cohen et al.\ (1996) taken with the 100 cm Lick reflector. This spectrum was taken in December 1987 and showed H$\alpha$ in emission: from the published plot we derived an approximate equivalent width of 7 \AA, comparable to our measures.

The dates of our spectroscopic observations are also marked in the bottom of Fig.~1 to better put them in the context of the stellar light-curve.   

Characteristic spectra at different epochs are reported in Fig.~5. All the spectra are typical of an N type giant, moderately obscured by dust in the circumstellar envelope, with the blue region strongly underexposed.
  
\IBVSfig{10cm}{6254-f5.eps}{Optical spectra of CGCS 6107 at different dates and luminosities. 
The y axis represents relative intensities corrected for the atmospheric extinction. The spectra are normalised at 7800 \AA. The main molecular bandheads are color-coded: blue = C$_2$; red = CN. 
The telluric bands of O$_2$ and H$_2$O molecules, overlapped to the CN red system, have not been removed.
The spectra are vertically shifted to each other for ease of comparison. From bottom to top: 2015-11-07 (r$\sim$11.4), 2016-12-02 (r$\sim$12.8), 2018-04-21 (r$\sim$13.0). }
\IBVSfigKey{6254-f5.eps}{CGCS 6107}{spectrum}
  
  
Red-ward of 5000 \AA{} the molecular absorption bands of  C$_2$ (Swan) and the red system of CN can be easily identified\footnote{most notably $\lambda\lambda$ 5636, 6122 and 6192 \AA{} of C$_2$;  5730, 5746, 5878, 6013,  6206, 6360, 6478, 6631, 6925, 7088, 7259, 7437, 7876-7945 and 8150 \AA{} of CN.}.
The 6260 \AA{} of the  C$_{13}$N$_{14}$~ is clearly visible  in the bright states;  the two absorptions of  atomic lines of K  at 7665, 7699  \AA{} are always  visible.
The  5889-5895 \AA{} NaD absorption is  possibly produced in the circumstellar envelope. The Balmer H$\alpha$ emission line is also recorded  with different intensities in different epochs.
 
Spectral changes are  correlated with the optical flux:  the continuum and the strength of H$\alpha$ and of the absorption bands are always affected by the veiling effect, mainly during faint photometric phases.
 H$\alpha$ emission was present at the end of 2015, the beginning of our monitoring, when the star was in bright state; it  was not present one year later, during a faint state; again  the emission was present near the next maximum, disappeared again when faint and rose again during the more recent brightening.
In the fainter states (December 2016 and January 2018) the depth of the molecular absorption bands was also reduced, while the equivalent width of the NaD lines in absorption did not vary significantly.


\begin{table}
\begin{center}
\caption{Spectroscopic observations logbook.}
\begin{tabular}{l c r c c c }
\hline
Telescope & date & res. (\AA) & r & H$\alpha$  (\AA) \\
\hline
Cima Ekar & 2015-11-07 & 8.0  & 11.4 & $-$8.3 \\
Cima Ekar & 2015-11-15 & 8.0  & 11.7 & $-$7.9 \\
Cima Ekar & 2015-12-20 & 8.0  & 11.7 & $-$7.8 \\
Loiano       & 2016-01-15 & 10.0  & 11.6 & $-$7.2  \\
Loiano       & 2016-12-02 & 10.0 & 12.8 & -----  \\
Loiano       & 2017-02-14 & 10.0 & 12.4 & $-$4.8    \\
Pennar      & 2017-04-06 & 6.9  & 12.5 & $-$5.6   \\
Pennar      & 2018-01-27 & 6.9  & 13.7 & -----  \\
Pennar      & 2018-04-13 & 6.9  & 13.0 & $-$5.7  \\
Pennar      & 2018-04-21 & 6.9  & 13.0 & $-$5.9  \\
Lick           & 1987-12-XX & 11 &  V=17.3: & $-$7 \\
\hline
\end{tabular}
\end{center}
\end{table}


%\vskip 1 cm



\section{Conclusions}

We have found that the variability of the carbon star CGCS 6107 is compatible with a quasi regular periodicity  on a time scale of about 543 days; the star may be classified as a SR variable because its average magnitude in each cycle is not constant. Historic observations  from DSS plates also show large variability. 

A definite change of the color indices ($V-r$) and ($r-i$) was detected, with the source being bluer when brighter. The H$\alpha$ line was in emission during maxima while disappeared in the fainter parts of the light curve: this is not unusual among AGB carbon stars. Overall the photometric and spectroscopic properties are similar to those of other variable carbon stars also studied by our group, like BIS 036 (HP Cam) or BIS 184 (Gaudenzi et al.\ 2017).

The absolute K magnitude of CGCS 6107 may be estimated from the relation (Whitelock et al.\ 2012):
 $$M(K) = -3.69\times( \log P - 2.38) - 7.18 (\pm0.37)$$ 
 which yields $M(K)= -8.35$: this gives an estimated distance of 4.9 kpc, with a probable range 5.8-4.2 kpc. The total galactic absorption in the $K$ band in the direction of the star is 0.13, much less than the uncertainty on the actual average $K$ magnitude of the star, given its variability.

The Gaia DR2 catalog (Gaia collaboration 2018), just published when we were finishing this paper, gives a parallax of 0.270 ($\pm$0.104) mas, corresponding to a distance of 3.7 ($-$1.0; +2.1) kpc, in fair agreement with our estimate.

\vskip 1.5cm

\noindent\textit{Acknowledgements:} We thank the Padova and Bologna Observatories for the time allocations. This work has made use of the VIZIER, SIMBAD, IRSA, VSX, ASAS-SN, STScI and Gaia DR2 databases.

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