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\IBVShead{4580}{8 April 1998}

\IBVStitle{PZ M\lowercase{on} - An Active Evolved Star}


\IBVSauth{STEVEN H.~SAAR}

\IBVSinsto{Smithsonian Astrophysical Observatory, MS-58}
\vskip -1mm
\IBVSinsto{60 Garden St., Cambridge, MA, USA~~~e-mail: saar@cfa.harvard.edu}

\SIMBADobj{PZ Mon}
\IBVStyp{ RS }
\IBVSkey{spectroscopy}
\IBVSabs{An Hipparcos parallax and new high-resolution spectra show that PZ Mon,}
\IBVSabs{long considered a spotted UV Ceti flare star, is actually a distant active} 
\IBVSabs{giant (K1III+?), probably an RS CVn, with $v \sin i \approx 10$ km s$^{-1}.$}

\begintext

PZ Mon is a relatively little studied active K2 star, of recent
interest due to discovery of a large amplitude ($\Delta m_{\rm B}
\approx 1.0$) spot cycle with a period $P_{\rm cyc} \approx 50$ years
(Bondar' 1995).  Its spectral type is given as K2Ve by Kukarkin (1958).  
First noted as a result of its variable optical
emission lines (Munch \& Munch 1955), PZ  Mon was classified a UV Ceti
flare star by Petit (1959). Photometric monitoring (Cristaldi \&
Rodon\`o 1970, 1973), however, showed that flaring is quite infrequent and
typically low amplitude.  Longer term brightness variations also
suggested it was a BY Dra variable (Wachmann 1968; Bondar' 1992).  
The star has been detected with $Einstein$ (Agarwal et al. 1986) and IUE
data (LWP14570L) show a saturated Mg II line.


In the course of a recent study of stellar dynamo cycles (Saar \&
Brandenburg 1998), we became interested in learning more about the star.  
Hipparcos data (ESA 1997) give a parallax of $\pi = 0.71 \pm 1.17$ mas, or a 
distance, $d \approx 1410^{+\infty}_{-880}$ pc.  The maximum $m_{\rm
pg} \approx  9.8$ and B$-$V = 1.10 (Bondar' 1995) implies V$_{\rm max}
\approx 8.8$ (assuming $m_{\rm pg}$ = B$-$0.11), consistent with the recent 
(near maximum) Hipparcos value of V = 9.03.  Combining V$_{\rm max}$ and 
$\pi$ yields $M_{\rm V} \approx -1.9$ (for $d =$ 1410 pc), indicating PZ 
Mon is clearly not a dwarf; rather, the $M_{\rm V}$ and B$-$V suggest a 
spectral type of $\approx$K0 II (e.g., Allen 1973; Gray 1992). The radius 
$R$ can be expressed as $\log R = \log d - 0.164 - 2 \log T_{\rm eff} - 
0.2$(V + BC) (Oranje 1986), yielding $R/R_\odot \approx$ 45 for 
$T_{\rm eff} \approx 4500$ K and $d = 1410$ pc (using the updated bolometric 
corrections of Flower 1996).  The large errors in $d$, however, mean 
$M_{\rm V} \approx 0.2$ (for $d =$ 530 pc) is also possible, which would 
make PZ Mon $\approx$K1 III with $R/R_\odot \approx$ 17.

%derived from the ratio of the surface fluxes at the star and earth
%$\log (F/f) = 2 \log (d/R) = 0.328 + 4 \log T_{\rm eff} + 0.4 ($V + BC),

To further explore PZ Mon's evolutionary state and other properties, we
obtained high resolution ($\lambda/\Delta\lambda = 125,000$, 2 pixel),
moderate S/N ($\sim 100$) spectra of PZ Mon and comparison stars with
the stellar echelle spectrograph and TI CCD detector (Smith \& Giampapa
1987) at the McMath--Pierce solar telescope of the National Solar
Observatory.  We used the 10 slice image slicer and the 180 mm transfer
lens; the resulting spectra cover a 20\AA~interval centered near 6170 \AA.
The data were dark subtracted, flat-fielded, optimally extracted,
and wavelength calibrated (using a quadratic fit to six Th-Ar lines). 


%and macroturbulent velocities ($v_{\rm mac}$).  In brief, this technique
%uses a simple Milne--Eddington line model whose fundamental parameters
%are determined from fits to lines in the Kurucz et al. (1984) solar
%flux atlas.  The model is integrated over the stellar disk using 15
%limb angles (with equal projected area; Bruning 1984) and 60 azimuthal
%sectors.  Values for $v \sin i$, $v_{\rm mac}$, and the line strength
%($\log gf$) are then varied using a conjugate gradient algorithm until
%the best fit is achieved.

Fig. 1 shows the PZ Mon data compared with a dwarf of similar color (HD
32147, K4V, B$-$V = 1.06) and a somewhat warmer RS CVn ($\lambda$ And,
G8 III-IV, B$-$V = 1.01).  We convolved the HD 32147 ($v \sin i <
1$ km s$^{-1}$; Saar \& Osten 1997) spectrum to $v \sin i = 10$ km
s$^{-1}$ with a rotational broadening function ($G(v \sin i)$; e.g.,
Gray 1992).  The $\lambda$ And spectrum was deconvolved by $G(6.5)$ to
correct for its $v \sin i = 6.5$ km s$^{-1}$ (Donati et
al.~1995), convolved with $G(10)$, and filtered to suppress
deconvolution noise. The wing of the strong 6162\AA~Ca I line is
gravity sensitive, and clearly $\lambda$ And is a better match near 
there than HD 32147.  Consistent with its higher $T_{\rm eff}$, line strengths 
in $\lambda$ And are on average slightly weaker than in PZ Mon.  The
radial velocity of PZ Mon at the midpoint of observations on HJD
2447834.997 was $v_r = +28.9 \pm 0.3$ km s$^{-1}$, based on
cross-correlation of the spectrum with one of HD 32147 ($v_r = +22.2$ 
km s$^{-1}$; Eggen 1992).  There is no
sign of a secondary star in our spectrum (Fig. 1); if present, it 
must have a flux $<$5\% of the primary at 6170 \AA.

Since PZ Mon is evolved, active, rapidly rotating (for a K giant/bright
giant) and spotted, it seems likely to be an RS CVn system.  The
K1III+? classification (putting $d \sim 500$ pc) is then perhaps
slightly preferred, since few RS CVn systems are bright giants
(Strassmeier et al.~1993).
Further $v_r$ data can help determine whether it is a binary.

% *** check - slt. shift to right?

% 10cm
\IBVSfig{8cm}{4580-f1.ps}{
McMath--Pierce data of PZ Mon (+; 0.0245 \AA~pixel$^{-1}$, $\lambda/\Delta 
\lambda = 125,000$, S/N $\approx$ 100) compared with HD\,32147 (K5V; heavy 
solid) and 
$\lambda$ And (G8III-IV; thin solid), both convolved to $v \sin i = 10$ km 
s$^{-1}$). All spectra have been shifted to the laboratory $\lambda$ scale. }

Since neither of the comparison stars was a perfect match, we also fit
selected line profiles using a simple radiative transfer model (Saar \&
Osten 1997) to confirm the $v \sin i$. The  
average results for 4 relatively unblended lines (Fe I 6165, 6180 \AA,
Ca I 6166 \AA, Ni I 6175  \AA; Fig. 2) was $v \sin i = 10.2 \pm
0.4$ km s$^{-1}$, close to results using a comparison star
(Fig. 1), and a
(radial-tangential) macroturbulent velocity $v_{\rm mac} = 5.5 \pm 0.8$
km s$^{-1}$. This $v_{\rm mac}$ is normal for a K1 III star 
($\langle v_{\rm mac} \rangle 
\approx$ 5.0 km s$^{-1}$; Gray 1992, his Fig. 18.9), though perhaps slightly
enhanced due to activity (Saar \& Osten 1997).  The magnetically
sensitive (Land\`e $g_{\rm eff}$ = 2.5) Fe I 6173\AA~line is best fit
by $v \sin i = 9.9$ km s$^{-1}$ and $v_{\rm mac}$ = 7.1 km s$^{-1}$;
the excess broadening in the wings (enhanced $v_{\rm mac}$) 
suggests the presence of significant magnetic flux (cf. 
Saar \& Linsky 1986).


% 10cm
\IBVSfig{7.8cm}{4580-f2.ps}{
McMath--Pierce data of PZ Mon (+) near the Fe I 6165.3 \AA~and Ca I 6166.4 
\AA~lines, fit with a simple radiative transfer model (solid), with residuals 
offset below.  Average results for four lines are
$v \sin i = 10.2 \pm 0.4$ km s$^{-1}$, and $v_{\rm mac} = 5.5 \pm 0.8$ km 
s$^{-1}$; $\sigma_{\rm fit}$ = 0.011. }

%  Enter input rate > .021
% * For Raymond Smith model with kT=  0.8617 keV (logT=7.0); NH =  2.000E+21
%   and  2.100E-02 cps in EINSTEIN IPC
%   (Model normalization =  1.374E-02)
% * PIMMS predicts a flux (  0.500-  4.500keV) of  4.764E-13 ergs/cm/cm/s

%  Enter input rate > 0.021
% * For Raymond Smith model with kT=  0.8617 keV (logT=7.0); NH =  2.000E+21
%   and  2.100E-02 cps in EINSTEIN IPC
%   (Model normalization =  1.374E-02)
% * PIMMS predicts a flux (  0.200-  4.000keV) of  4.790E-13 ergs/cm/cm/s


Agarwal et al.~(1986) detected PZ Mon with $Einstein$ at 0.021 IPC
counts s$^{-1}$. The star appears to be variable in X-rays:
SIMBAD lists a detection at 0.014 IPC counts s$^{-1}$.  Assuming a
coronal temperature of $10^7$ K and an ISM column of $\log n_H \approx
21.32$ (equal to HD\,48279, 1\fdeg 4 distant at $d = 1635$
pc; Shull \& Van Steenberg 1985), using PIMMS, the Agarwal et al.~flux 
works out to $f_X \approx 5
\times 10^{-13}$ ergs cm$^{-2}$ s$^{-1}$ (0.2-4.0 keV) at earth.  
Using the calibration of (Oranje 1986), the ratio of
surface fluxes (at star/at earth) is $\log F/f \approx 18.3$,
implying $\log F_X$ =  6.0 ergs cm$^{-2}$ s$^{-1}$ and $\log F_X/F_{\rm bol}$ = 
$\log L_X/L_{\rm bol}$ = $-$4.2, and $\log
L_X = 31.8$ ergs s$^{-1}$ (if $d \approx$ 1410 pc) or $\log L_X = 
30.9$ ergs s$^{-1}$ (if $d \approx$ 530 pc).  These values are
typical of RS CVn systems (e.g., Dempsey et al.~1993).

The shift of PZ Mon from a dwarf to a giant classification 
is consistent with its very
low level of flaring (RS CVns show few optical flares; Henry \& Newsom
1996).  It is also consistent with its non-detection by the ROSAT EUVE 
survey (Tsikoudi
\& Kellett 1997) -- the larger ISM column due to greater $d$ would absorb
much of the EUV flux.  The combined properties of the star suggest it may be
a distant analog to $\sigma$ Gem (K1III+?, B$-$V = 1.12, $v
\sin i = 25$ km s$^{-1}$, $\log L_X \approx$ 31.4; Strassmeier et al.
1993). As PZ Mon is now an active giant, V833 Tau (Hartmann et al. 1981;
Bondar' 1995) appears to reclaim the title of the dwarf star with the
largest photometric spot cycle amplitude.  PZ Mon becomes one of the
relatively few evolved stars with a well determined starspot cycle. 

In summary, an Hipparcos parallax and new high-resolution spectra
show that PZ Mon, long considered a spotted UV Ceti flare star, is
actually a distant active giant (probably $\sim$K1III+?) with $v \sin i
\approx 10$ km s$^{-1}$ and $v_r = +28.9$ km s$^{-1}$ on HJD
2447834.997.  It is likely an RS CVn variable, similar to $\sigma$
Gem in many respects.  The recalibrated X-ray properties of PZ Mon
are consistent with RS CVn systems.  \\

 


I thank F.R. Harnden and J. Bookbinder for comments, 
the NSO for generous allocation of telescope time, and Dave Jaksha for 
superb technical support.  The NSO is run by AURA, Inc. for the NSF.
This research is funded by NSF grant AST-9528563 and 
made use of the SIMBAD database, operated at CDS, Strasbourg, France. \\


\vskip 1cm


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