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\IBVShead{4987}{21 November 2000}

\IBVStitletl{Str\"{o}mgren Photometry of the T Tauri Star}{ SU Aurigae: Multi-timescale Light Variations}

\IBVSauth{Nadalin, I.; DeWarf, L.E.; Guinan, E.F.}

\IBVSinsto{Villanova University, Villanova, PA, USA, e-mail: ldewarf@email.villanova.edu}

\SIMBADobjAlias{SU Aur}{HD 282624}
\IBVStyp{ CTTS }
\IBVSkey{photometry}
\IBVSabs{Intensive photoelectric photometry from October 1993 to March 2000.}
\IBVSabs{Analysis of "eclipse-like" dimming events and rotation period}
\IBVSabs{determination from short period mean light modulations.}

\begintext

Intensive long-term photoelectric photometry of classical T Tauri stars
(CTTS) can provide fundamental new information that helps to understand
stars in the early stages of evolution.  CTTS typically have spectral
types from F to K, exhibit weak H$\alpha$ and Ca\,II\,H\,\&\,K
emissions, have broad absorption lines (implying rapid rotation), and
are located well above the main-sequence.  One of the brightest
archetypical CTT stars, SU Aurigae (HD 282624; G2 IIIe; $\langle
V\rangle = +9\fmm20$; $\langle B-V\rangle = +0\fmm90$) is observed to
undergo rapid and dramatic light variations.  These light variations
appear not to be accompanied by spectral changes (Herbst \& Shevchenko,
1999).  This implies possible obscuration of the star by dust with
properties similar to the interstellar medium (ISM).  In a previous
paper by the authors (DeWarf {\it et al.}, 1998; hereafter Paper I),
the interstellar absorption of SU Aur was determined, and when combined
with the Hipparcos distance, yielded estimates of its absolute
magnitude and intrinsic colors.  These values placed SU Aur about 1.8
mag above the main-sequence for its respective unreddened spectral
type.  In Paper I, SU Aur was plotted on pre-main sequence (PMS)
evolution tracks and an age of about 4 Myrs and a mass of $1.9 \pm
0.1~M_\odot$ was ascertained.  The observations presented here were
made using the 0.8-m Four College Automatic Photoelectric Telescope
(APT), in Arizona.  This concentrated (nightly) photometry, has began
in October 1993 and continuing to the present, is made in the
Str\"{o}mgren $uvby$ system.  A description of the instrumentation,
observing, and reduction procedure is given in Paper I.

After analyzing the data collected since 1993, large ($\Delta V \approx
\Delta y \approx 0\fmm40$) ``eclipse-like'' dimming events have been
observed frequently.  Representative photometry from the 1998/99
observing season is shown in Fig.~1.  In the figure the Str\"{o}mgren
$b$ magnitudes and the corresponding Str\"{o}mgren [$c_1$] and [$m_1$]
indices (Str\"{o}mgren, 1966; Crawford \& Mandwewala, 1976) are plotted
against Heliocentric Julian Day.  As shown, the light variations of SU
Aur, like many T Tauri stars, are complicated.  In addition to the
short-term ``dips'' in the light curve, the star also varies on time
scales of days, months, and years.  Excluding the dips, the overall
light variations in $b$ are between $b_{\rm max}\simeq +9\fmm75$ to
$b_{\rm min} \simeq +10\fmm20$ over September 1998 to March 1999.  As shown in
the upper panel of Fig.~1, four large dimming events were observed
nearly 48 days apart, lasting several days each.  To understand the
possible cause of such drops, the reddening-independent [$c_1$] and
[$m_1$] indices  were calculated.  For data from 1993 to the present we
find average values of $0\fmm30\pm0\fmm060$ and $0\fmm50\pm0\fmm087$
for [$c_1$] and [$m_1$] respectively.  As seen in Fig.~1, these indices
do not show any significant variation, though the $b$ filter light
varies by more than 0\fmm75.  It is therefore reasonable to conclude
that orbiting concentrations of dust could produce the observed dimming
events, and that this dust has properties similar to that of the ISM.
CTT stars are pre-main sequence objects that have extensive accretion
disks and SU Aur has a large infrared excess implying a large
circumstellar cloud.  Hence, it is possible that these dimming events
were caused by concentrations of matter orbiting in the outer regions
of the disk, by clumps of material surrounding a forming planet, or
from dust clouds condensing from ejected matter and, in the process,
obscuring the star.

\IBVSfig{9.9 cm}{4987-f1.eps}{The 1998/99 $b$ filter 
light curve for SU Aur is shown in the top panel with the 
Str\"{o}mgren $[c_1]$ and $[m_1]$ indices in 
the lower panel.  The reference lines in the lower panel 
represent the average values since 1993.  Although there are 
dramatic drops in mean light, the indices remain relatively 
unaffected.  This implies obscuration by dust with properties 
similar to ISM dust.  We find average maximum light levels 
during this epoch to be about 12.0, 10.8, 9.9, 9.4 mag 
for $u$, $v$, $b$, and $y$.}

The accretion disk and the possible concentrations of matter orbiting
around SU Aur are likely being observed nearly edge-on. From speckle
interferometry taken along a N-S position angle (DeWarf \& Dyck, 1993),
the star appears to have no measurable projected spatial extent beyond
about 29 AU in the $L'$ band (3.8 $\mu$m).  Using the IRAS fluxes
(IRAS Point Source Catalog; {\it Joint IRAS Science Working Group},
1988), the spectral slope as defined by Adams {\it et al.}\ (1987)
$$
\eta  = \frac{d \log\nu F_{\nu}}{d \log\nu}
\quad \Longrightarrow \quad
\eta_{12}  \approx \frac{\log\displaystyle \frac{\nu_{12}F_{12}}
{\nu_{25}F_{25}}}{\log\displaystyle \frac{\nu_{12}}
{\nu_{25}}}
$$
is $-0.65$ for the spectral region between 12 and 25 $\mu$m.  Examples
of stars that have similarly red spectral slopes are AFGL490, Elias 29,
and HL Tau.  These stars have resolved infrared emitting circumstellar
material projected out to about 1100, 600, and 200 AU respectively
({\it cf\/} DeWarf \& Dyck, 1993).  Of course, as Adams {\it et al.}\
(1988) has shown, for circumstellar disk geometry, a high inclination
angle will result in significant reddening.  HL Tau is an excellent
potential candidate for this type of geometry (Lay {\it et al.}, 1997;
Mundy {\it et al.}, 1996; Sargent \& Beckwith, 1991).  Therefore, it is
likely that for HL Tau and SU Aur, the actual (not just projected)
extent of the circumstellar material is less than the extent of the
material around either  AFGL490 or Elias 29. The results of the speckle
interferometry and particularly red spectral slope imply that the
infrared emitting region of SU Aur is confined to a disk, and that the
disk is observed nearly edge-on.

\IBVSfig{13.6 cm}{4987-f2.eps}{Three relatively constant 
sections of the SU Aurigae light curve.  The rotational 
modulation of light due to starspots can be modelled by 
applying a direct (grid search) parameter optimization 
procedure.  In all three cases a period of about 1.7 days 
was determined.}

To find the rotation period of SU Aur from photometric data,
appropriate sections of the light curve needed to be isolated.  The
rotation period is determined from the short-term, low amplitude,
sinusoidal-like modulations in brightness due to the rotational effects
of starspots.  Many studies have established the rotation periods of T
Tauri stars in this manner (see, for example, Bertout, 2000;
Strassmeier {\it et al.}, 1999).  Suitable portions of the light curve
would each need to be $\sim 10$ days long --- long enough to contain
sufficient observations, but short enough to minimize the effects of
possible spot creation, destruction, or migration.  The sections
selected also need to be relatively constant to avoid any rapid or
significant drop in flux levels due to obscuration.  Examination of our
observations revealed three such regions, shown in Fig.~2.  Using a
straightforward sinusoidal model, we applied a direct parameter
optimization procedure to find the period of rotation.  This method is
analogous to an iterative grid search in which each of the independent
parameters are varied separately until the minimum in the sum of the
deviations is achieved.  A period of approximately 1.7 days was found
for each of these sections.  For the $b$ observations, we find the full
range of the light modulations to be 0\fmm064, 0\fmm098, and 0\fmm084 for
the 1995, 1998, and 1999 sections shown in Fig.~2.  Hartmann {\it et
al.}\ (1986) report a projected rotational velocity ($v\sin i$) of
$66.2\pm4.6$ km/sec.  From this high rotational velocity and the
speckle interferometry results, we can assume that the inclination
($i$) is close to 90\deg.  Therefore, the radius of the star can be
determined,
$$
R_{\rm rot} \sin i = P_{\rm rot} v (\sin i) / 2 \pi \simeq 2.2~R_\odot.
$$
Given that $L=5.1~L_\odot$ and $T_{\rm eff}=5550$~K 
(Paper I), the above value can be compared to the photometrically 
determined radius,
$$
R_{\rm phot} = \sqrt{L / (4 \pi \sigma T_{\rm eff}^{4})} \simeq 2.5~R_\odot.
$$
Since the visible light emissions come from the 
star and reprocessed light from the extended circumstellar 
material, the photometrically determined overall luminosity 
should be larger than the actual stellar luminosity.  Also, 
the enshrouding dust would redden, and therefore possibly 
lower, the photometrically determined effective temperature.  
Hence, the photometrically determined radius should be 
expected to be larger than the actual radius of the star.

This research is supported by NSF/RUI Grants AST-93 15365 
and AST-00 71260, which we gratefully acknowledge.


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\end{document}
