---
author:
- 'M. Gromadzki'
- 'J. Miko[ł]{}ajewska'
- 'P.A. Whitelock'
- 'F. Marang'
bibliography:
- '6538ref.bib'
date: 'Received; accepted ??'
title: On the nature of the cool component of MWC 560
---

[MWC 560 (V694 Mon) is one of the most enigmatic symbiotic system with a very active accretion-powered hot component. Such activity can be supported only by a luminous asymptotic giant branch star, i.e. a Mira or SR variable, with a high mass-loss rate. It is also a very unusual jet source because the jet axis lies practically parallel to the line of sight. ]{} [The aims of our study are the determination of the evolutionary status of the cool component of MWC 560.]{} [Our methods involve analysis of near-IR $JHKL$ and optical light curves.]{} [The cool component of MWC 560 pulsates with a period of $\sim$340 days, and it is probably a red SR variable on the thermally pulsing AGB. The high mass-loss rate expected for such a star is sufficient to power the observed activity of the hot companion.]{}

Introduction
============

Discovered by @Merrill1943 as a star with strong emission lines, MWC 560 (V694 Mon) is an enigmatic symbiotic system with a very active hot component. Its optical spectrum is always characterized by highly variable absorption features, blue-shifted by 1000-6000 $\rm km\,s^{-1}$. These originate from , , and , and are detached from the stationary, narrow emission lines. The blue-shifted absorption features can be explained by a jet outflow along the line of sight [@Toma1990]. In 1990, MWC 560 underwent a 2 mag photometric outburst, accompanied by spectacular changes in these blue shifted absorption lines [e.g. @Toma1990; @Schmid2001 and references therein]. The hot component is also a permanent source of rapid flickering with amplitudes in the range 0.2-0.7 mag on time scales of 10-100 min [e.g. @grom2006 and references therein].

Both the flickering and the 1990 outburst characteristics suggest that the hot component is predominantly powered by unstable accretion, and its luminosity $\sim$100-1000 $L_{\odot}$ [@Toma1996; @Mamiko1998] requires relatively high accretion rates of $\dot{M} \ga 10^{-7} M_{\odot} \mathrm{yr}^{-1}$ [@Schmid2001].

@Doroshenko1993 found a 1930-day period in the $m_{\rm{ph}}$ historical light curve [@Lut1991] combined with more recent $B$-band photometry. They suggested an orbital origin of this period: if the orbit is eccentric, near periastron the accretion rate increases, and the hot component gets brighter.

We know rather little about the cool component of MWC 560. However, the high activity of its companion requires a high mass-loss rate, consistent with an evolved AGB star rather than a normal red giant. Absorption features in the optical spectrum suggest an M3$-$M4 spectral type [@Sand1973; @Allen1978]. Infrared spectral classification mainly based on TiO bands, and the CaII triplet indicates an M4$-$M5 giant [@Szkody1990; @Bopp1990; @ThiW1992]. Based on the lack of VO bands at 1.05 $\mu$m @Meier1996 classified the cool component as an M5$-$M6 III giant. The most recent classification based on five TiO bands in the near infrared resulted in M5.5 and M6 (two different observations) [@Murset1999]. @Franckowiak2003 found possible pulsations of red giant with $P=161$ days, and concluded that it is an AGB star.

This paper contains an analysis of near infrared ($JHKL$) and visual (AAVSO and ASAS) light curves with the objective of searching for periodic changes and establishing the nature of the cool giant.

Observations
============

![Light curves for MWC 560. From top to bottom: AAVSO+ASAS and $JHKL$ light curves, and $J-K$ colours, respectively.[]{data-label="Figlc"}](6538fig1.eps){width="8.5cm"}

$JHKL$ broad-band photometry (1.25, 1.65, 2.2, 3.45 $\mu$m) was obtained with the MkII infrared photometer on the 0.75 cm telescope at SAAO, Sutherland (see @Carter1990 [-@Carter1990] for details about the system). The measurements are good to $\pm 0.03$ at $JHK$ and $\pm
0.05$ at $L$. The optical light curve consists of visual magnitude estimates collected by the American Association of Variable Stars Observers (AAVSO), and $V$-band photometry obtained in the frame of All Sky Automated Survey (ASAS) [@Pojmanski2002]. The near-IR observations cover the period from November 1984 to February 2004 (92 points in $J$ and $K$ bands, 91 in $H$ and 86 in $L$) whereas the optical photometry was collected between April 1990 and December 2001 - the AAVSO data (2051 points), and from November 2000 to February 2006 - the ASAS data (330 points), respectively. The near-IR data are listed in Table 1 and the light curves are presented in Fig. 1.

  ---------- ------ ------ ------ ------
      JD      $J$    $H$    $K$    $L$
   -2440000                       
    (day)                         
   6024.55    6.50   5.49   5.12  
   6026.45    6.51   5.50   5.14   4.74
   6034.53    6.48   5.49   5.12   4.79
   6043.57    6.49   5.48   5.13   4.79
   6082.46    6.54   5.52   5.16   4.84
   6108.39    6.54   5.53   5.18   4.84
   6113.33    6.56   5.56   5.19   4.86
   6136.30    6.54   5.52   5.16   4.85
   6184.24    6.49   5.44   5.10   4.90
   6338.57    6.50                
   6380.61    6.49   5.45   5.12   4.73
   6440.46    6.59   5.54   5.20   4.90
   6454.48    6.56   5.53   5.17   4.89
   6465.35    6.54   5.50   5.16  
   7958.39    6.43   5.45   5.11   4.76
   7961.37    6.42   5.44   5.11   4.80
   7964.33    6.43   5.43   5.10   4.75
   7989.26    6.44   5.42   5.09   4.78
   8023.22    6.42   5.41   5.06  
   8028.19    6.39   5.42   5.08   4.71
   8197.59    6.47   5.49   5.16   4.84
   8227.56    6.48   5.49   5.16   4.79
   8252.56    6.48   5.49   5.15   4.82
   8280.49    6.43   5.44   5.11   4.71
   8294.38    6.43   5.45   5.11   4.74
   8320.40    6.44   5.44   5.10   4.80
   8326.41    6.47   5.45   5.11   4.79
   8390.20    6.44   5.43   5.08   4.74
   8617.53    6.44   5.42   5.08   4.77
   8670.41    6.48   5.41   5.07   4.69
   8705.33    6.49   5.41   5.05   4.70
   8731.29    6.42   5.37   5.03   4.69
   8768.20    6.45   5.44   5.09   4.74
   8899.63    6.49   5.48   5.14   4.82
   8932.56    6.47   5.45   5.15   4.85
   8960.52    6.44   5.43   5.10   4.73
   8968.44    6.42   5.41   5.06   4.71
   8969.52    6.42   5.43   5.09   4.79
   8991.41    6.44   5.40   5.06   4.70
   9024.42    6.43   5.41   5.06   4.77
   9292.54    6.47   5.43   5.09   4.79
   9379.36    6.38   5.36   5.01   4.67
   9503.19    6.52   5.50   5.17   4.80
   9643.62    6.53   5.52   5.15   4.83
   9667.53    6.51   5.49   5.15   4.82
   9728.50    6.47   5.44   5.10   4.72
   10035.54   6.48   5.47   5.13   4.81
   10053.60   6.46   5.44   5.10   4.73
   10086.47   6.50   5.48   5.13   4.79
   10110.42   6.52   5.49   5.14   4.77
   10126.45   6.52   5.50   5.15   4.87
   10181.29   6.46   5.46   5.11   4.74
   10221.22   6.45   5.43   5.09   4.72
   10437.54   6.45   5.44   5.10   4.78
   10464.46   6.49   5.45   5.11   4.79
   10478.37   6.49   5.46   5.12   4.81
   10498.39   6.48   5.47   5.14   4.86
  ---------- ------ ------ ------ ------

  : IR photometry of MWC 560

  ---------- ------ ------ ------ ------
      JD      $J$    $H$    $K$    $L$
   -2440000                       
    (day)                         
   10567.23   6.50   5.47   5.14   4.77
   10801.51   6.54   5.49   5.14   4.81
   10884.38   6.47   5.42   5.09  
   11146.59   6.48   5.47   5.11   4.86
   11187.48   6.51   5.48   5.13   4.72
   11234.30   6.46   5.43   5.08  
   11479.54                 5.14  
   11505.51   6.53   5.48   5.13   4.77
   11511.48   6.52   5.46   5.12   4.81
   11572.48   6.52   5.46   5.11  
   11599.35   6.50   5.46   5.12   4.82
   11611.33   6.52   5.48   5.12   4.78
   11617.34   6.52   5.49   5.15   4.76
   11675.24   6.50   5.46   5.11   4.76
   11857.58   6.54   5.52   5.17   4.86
   11864.58   6.55   5.54   5.19   4.90
   11869.52   6.56   5.52   5.18   4.87
   11881.54   6.54   5.51   5.18   4.87
   11890.57   6.55   5.52   5.18   4.85
   11927.47   6.54   5.52   5.18   4.88
   11976.31                        4.85
   12239.58   6.49   5.47   5.13   4.75
   12255.57   6.53   5.49   5.15   4.80
   12263.47   6.51   5.50   5.16   4.75
   12284.50   6.52   5.49   5.13   4.76
   12289.44   6.51   5.48   5.16   4.81
   12319.40   6.48   5.45   5.12   4.74
   12324.44   6.49   5.47   5.10   4.80
   12349.32   6.48   5.44   5.10   4.80
   12384.26   6.49   5.43   5.09   4.85
   12572.58   6.59   5.53   5.17   4.87
   12594.60   6.56   5.52   5.15   4.81
   12664.43   6.52   5.50   5.15   4.87
   12690.38   6.52   5.45   5.09   4.79
   12979.55   6.49   5.45   5.12   4.79
   13031.48   6.48   5.41   5.06   4.73
   13063.34   6.41   5.37   5.03   4.66
  ---------- ------ ------ ------ ------

  : continued.

Period Analysis
===============

We analysed the near-IR, ASAS, and combined AAVSO+ASAS magnitudes by means of the program PERIOD[^1] ver. 5.0, which uses the modified Lomb-Scargle method [@PressandRybicki1989]. To remove long-term trends from the optical data, a second order polynomial was subtracted from the combined AAVSO+ASAS light curve, and a third order spline function was subtracted from the ASAS light curve. The resulting power spectra are shown in the right panel of Fig. 2, and results of our period analysis are summarized in Table 2. The periods given in the table were derived from the maxima of the peaks in the periodograms ($f_{max}^{-1}$), whereas their accuracy was estimated by calculating the half-size of a single frequency bin ($\Delta f$), centred on the peak ($f_{c}$) in a periodogram, and then converted to period units ($\Delta P = f_{c}^{-2} \cdot \Delta f$).

Power spectra of the composite AAVSO+ASAS light curve (Fig. 2, the uppermost, right panel) show a very pronounced peak at $1931^{\rm{d}}$ ($P_\mathrm{act}$) very close to the period reported by @Doroshenko1993. The second and third harmonics, $\sim$1931/2 and $\sim$1931/3, respectively, are also present. We attribute this periodicity to repeated episodes of enhanced activity of the hot component following the ephemeris: $${\rm{JD(max)}}=2\,448\,080 + 1931\times E$$ The light curve folded with this period is also show in Fig. 2 (the uppermost, left panel).

Power spectra corresponding to the $JHKL$ light curves are very similar to each other (Fig. 2). In all spectra the strongest peak corresponds to a 339-day period ($P_\mathrm{pul}$) which we take to be a real periodicity produced by pulsations of the cool component. There are also peaks around $\sim$4500-5500 days (which is comparable to the time interval spanned by the data) connected with the long-term trend. Power spectra of the $HK$ light curves show also peaks at $\sim1900^{\rm{d}}$ days and $\sim 310^{\rm{d}}$. The former is very close to $P_\mathrm{act}$ found in the optical light curve whereas the latter seems to be an annual alias of 1900-day peak rather than a real periodicity. In the power spectrum of the $J$ light curve the $\sim$1900-day and $\sim$310-day periods are not seen because they are overwhelmed by the strong long term trend. These periods are absent in the power spectrum of the $L$ light curve. The following ephemeris gives the phases of maxima in the $JHKL$ bands: $${\rm{JD(max)}}=2\,445\,960 + 339\times E$$ and the light curves folded with this ephemeris are shown in Fig. 2. (left panel).

The power spectra of the visual photometry also show peaks at $\sim
310^{\rm{d}}$. These power spectra have poorer resolution that those for the near-IR data, so the peaks, especially that for the ASAS data, are relatively broad, and they may in fact be due to the $339^\mathrm{d}$ pulsation period combined with one year alias of the $1931^\mathrm{d}$ period. Such an interpretation is supported by the presence in the ASAS power spectrum of another peak at $\sim
166^\mathrm{d}$. This peak can be more accurately determined and it is due to the second harmonic of the pulsation period, which in this case would be $332^\mathrm{d}$. The ASAS light curve folded with this period is also shown in Fig. 2.

In the power spectrum of the AAVSO+ASAS data there is also a strong peak at $747^{\rm{d}}$ connected with a quasi-periodic oscillation which is predominantly visible during JD 2450500-2452300 (1997-2002). However, we note that it is very close to 2 years, and it may be an artifact.

  ------------------ -------------- ------------ --------------------------
      Frequency          Period        Power              Remarks
     days$^{-1}$          days       sigma unit  
          J                                      
   2.9479 10$^{-3}$    339$\pm$4        20.8          $P_\mathrm{pul}$
                                                 
          H                                      
   5.3276 10$^{-4}$   1877$\pm$126      7.0           $P_\mathrm{act}$
   2.9479 10$^{-3}$    339$\pm$4        19.8          $P_\mathrm{pul}$
   3.2321 10$^{-3}$    309$\pm$3        10.6               alias
                                                 
          K                                      
   5.3276 10$^{-4}$   1877$\pm$126      7.2           $P_\mathrm{act}$
   2.9479 10$^{-3}$    339$\pm$4        20.2          $P_\mathrm{pul}$
   3.2676 10$^{-4}$    306$\pm$3        12.2               alias
                                                 
          L                                      
   2.9487 10$^{-3}$    339$\pm$4        11.6          $P_\mathrm{pul}$
                                                 
         ASAS                                    
   3.2708 10$^{-3}$    306$\pm$12       35.8         $P_\mathrm{ASAS}$
   6.0183 10$^{-3}$    166$\pm$4        18.5         $P_\mathrm{pul}/2$
                                                 
      AAVSO+ASAS                                 
   5.1789 10$^{-4}$   1931$\pm$162     285.6          $P_\mathrm{act}$
   9.4946 10$^{-4}$   1053$\pm$48       57.9         $P_\mathrm{act}$/2
   1.3379 10$^{-3}$    747$\pm$24      102.4     
   3.1937 10$^{-3}$    313$\pm$4        53.1      $P_\mathrm{ASAS}$, alias
  ------------------ -------------- ------------ --------------------------

  : Peaks in power spectra[]{data-label="table:2"}

![Power spectra (right) and corresponding light curves folded with the strongest period (left). From the top to bottom: AAVSO+ASAS light curve folded with period $P_\mathrm{act}$=1931 days, ASAS light curve folded with period $P_\mathrm{ASAS}$=332 days, and $JHKL$ light curves all folded with period $P_\mathrm{pul}$=339 days, respectively. []{data-label="Figfaza"}](6538fig2.eps){width="8.5cm"}

Discussion and conclusions
==========================

Our near-IR photometry began in 1984, with only a dozen measurements over a $~1.5$-year interval. The observations restarted in 1990, when MWC 560 underwent an outburst, reaching the brightest level in its whole photometric history, and ejected jets [@Toma1990]. In the near-IR, it brightened by $\sim$0.1 mag in all, $JHKL$, bands. Then during the following 12 years, it slowly faded to the pre-outburst magnitudes observed in 1984. In 2002-2004, the star brightened again. The point scatter of the near-IR light curves was always of $\sim$0.1 mag in $JHK$ bands, and $\sim$0.2 mag in $L$ band, respectively. In general, the near-IR light curves reflect the trend shown by the visual/$V$ light curve, with additional maxima around JD 2450000 and JD 2452000. The $J-K$ colour becomes redder as the brightness declines (Fig. 1) due to a decreasing contribution from the hot component.

Our period analysis of the $JHKL$ light curves revealed a 339-day periodicity which can be attributed to radial pulsations of the M giant. Although this period is close to that of Mira itself, and of many other Galactic Mira variables, the amplitude of the pulsation, $\Delta\,K
\sim 0.1$ mag, is much lower than $\Delta\,K \ga 0.4$ mag observed in Miras, including typical symbiotic Miras [@Whitelock1987]. We therefore classify it as an SRa variable (using the definition in the GCVS4) rather than as a Mira. This pulsation is hardly detectable in the visual light because radiation in this range is dominated by the very active hot component. In fact, the scatter of points from the folded light curve in Fig. 2, $\Delta V
\sim 0.3 \div 0.6$ mag is comparable to the amplitude of the flickering [@grom2006] which complicates the period analysis. The red giant pulsation period found in this study is more than twice the 161-day pulsation period reported by @Franckowiak2003, and it is the only periodicity present in all bands.

The basic properties and evolutionary status of the semi-regular variables (SRVs) of type SRa and SRb were discussed in detail by @Kerschbaum_and_Hron1992 [@Kerschbaum_and_Hron1994; @Kerschbaum_and_Hron1996]. In particular, they found that the SRas appear as intermediate objects between Miras and SRbs in all aspects, including periods, amplitudes and mass-loss rates. They also concluded that the SRas do not form a distinct class of variables, but are a mixture of ‘intrinsic’ Miras and SRbs. The SRbs split into a ‘blue’ group with $P<150$ days and no indication of circumstellar shells and a ‘red’ group with temperatures and mass-loss rates comparable to Miras, but periods about half as long. They suggested that the ‘red’ and ‘Mira’ SRbs are thermally pulsing AGB-stars [@Kerschbaum_and_Hron1992]. The persistent and relatively long period places the cool component of MWC 560 among the ‘Mira’ SRVs. These differ from normal Miras only in their smaller pulsation amplitudes. The IRAS \[12$\mu$m\]-\[25$\mu$m\]=0.64 locates MWC 560 in the period–IRAS colour diagram of Kerschbaum & Hron (1992) in the region occupied by 86 % of the Miras, whereas most SRbs are outside this region. The average, reddening corrected ($E_\mathrm{B-V}=0.15$; Schmid et al. 2001) near-IR colours $\langle H-K
\rangle_0=0.30$, $\langle J-H \rangle_0=1.00$, $K-[12\,\mu$m\]=1.25, and the spectral type M5.5$-$6 are also consistent with an O-rich SRV, although the colours are unlike those of a Mira. Finally, if the 1931 day periodicity is orbital then the two stars of this symbiotic system are close to each other (see below), and the presence of a nearby companion may influence the pulsation characteristics, possibly reducing the pulsation amplitude. We suggest that the evidence supports the view that the red component of MWC 560 is on the TP AGB-phase.

The SRV component of MWC 560 could be the source of a strong stellar wind and thus support the observed high activity of the hot component. If we estimate the mass of the red giant at $1M_{\odot}$ and that of the white dwarf companion at $0.5M_{\odot}$ then, assuming that the 5.3 year period is orbital, the separation of the two stars will be 3.55 AU and the radius of the Roche lobe for the M giant in a circular orbit will be 1.5 AU. @Schmid2001 argued that an accretion rate up to $\rm {a\,few} \times
10^{-7}\, M_{\sun}\, \mathrm{yr}^{-1}$ is necessary to account for the observed hot component luminosity which is comparable to the wind efficiency found for typical Miras and SRVs. For example, @Olofsson2002 determined average mass loss rate of $2 \times 10^{-7}\, M_{\sun}\,
\mathrm{yr}^{-1}$, with a maximum value of $8 \times 10^{-7}\, M_{\sun}\,
\mathrm{yr}^{-1}$ for a sample of M-type irregular and semi-regular variables. So, in the case of MWC 560, a significant fraction of the mass lost in the cool giant wind must be accreted by the hot companion. This is possible only if the binary components of MWC 560 interact via ‘wind Roche lobe’ overflow occurs instead of a spherically symmetric wind [@Podsiad2006].

One of the most intriguing features of this hot component activity is its highly periodic character. The 1931-day periodicity found in the visual AAVSO+ASAS light curve is essentially identical with the 1930-day periodicity detected in the $m_\mathrm{pg}/B$ light curve by @Doroshenko1993. We note that this periodicity remained in-phase over a century, and the most natural explanation would be an orbital motion. The only problem with the orbital interpretation is that the orbital inclination of MWC 560 seems to be extremely low (as indicated by the jet axis practically aligned with the line of sight, and the lack of orbitally related radial velocity changes) which excludes any geometrical effects, such as eclipses, illumination, etc. Although one can argue that the jet axis can be inclined with respect to the binary orbital plane, the relatively large amplitude of this variability, $\Delta m_\mathrm{pg} \sim 1$ and $\Delta V \sim 0.5$ mag, respectively, points to another mechanism(s). One possibility is an eccentric orbit and enhanced accretion rate near periastron passage causing a brightening of the hot component [@Doroshenko1993]. Then we can even speculate that if the eccentricity is high enough, $\sim 0.5$ or more, a Roche lobe overflow may occur near the periastron.

Another possibility is enhanced mass loss due to periodic changes in the SRV environment. We note that the 1931 day period equals roughly six times the pulsation period. A periodically enhanced mass loss might be caused by some kind of interplay between the cool component pulsation and layered dust formation shells. Such effects have been shown by time-dependent hydrodynamic simulation of C-rich Mira environments [@Winters1999; @Winters2001]. In particular, these simulations have been able to reproduce variations of the mass-loss rates on different time scales.

This study made use of the American Association of Variable Star Observers (AAVSO) International Database contributed by observers worldwide and the public domain database of The All Sky Automated Survey (ASAS) which we acknowledged. We also thank the following people for making IR observations: Greg Roberts, Robin Catchpole, Brian Carter, Dave Laney and Hartmut Winkler. This research was partly supported by KBN grant 1P03D01727.

[^1]: source of program is available on [*http://www.starlink.rl.ac.uk/*]{}
