A&A 388, 504-517 (2002)
DOI: 10.1051/0004-6361:20020413
J. F. Gameiro 1,2 - D. F. M. Folha 1 - V. M. Costa 1,3
1 - Centro de Astrofísica da Universidade do Porto, Rua das
Estrelas, Porto 4150, Portugal
2 -
Departamento de Matemática Aplicada, Faculdade de
Ciências da Universidade do Porto, Portugal
3 -
Departamento de Matemática, Instituto Superior de
Engenharia do Porto, Portugal
Received 12 December 2001 / Accepted 11 March 2002
Abstract
We report optical spectroscopic and photometric results from
our long-term study of the T Tauri star WY Arietis (LkH 264). The data
gathered show different types of variability: variations in the
continuum level, in the emission line fluxes and line profiles. The
timescales associated with these variations appear quite diverse. The
correlation found between the variations observed in the veiling and in
the continuum flux strongly suggest that an extra continuum source
veiling the stellar photospheric spectrum is the cause driving the
continuum variability. The present work also unveils the presence of an
accretion flow onto the star, as revealed by the O I
Å and
Å line profiles, which is the first
unambiguous model-independent detection of such an event in this
star. Our photometric data allowed us to find a period of 3.04 days for
this star, somewhat in tune with Fernandez & Eiroa
(1996). However, due to the poor time sampling our
finding should be taken as tentative. A detailed analysis of the broad
and narrow components of the He I line profiles indicates the presence
of a hot wind during the November 1993 observation while in October 1999
a wind is only revealed by the blue wing asymmetry of the observed
Balmer and CaII infrared triplet line profiles. The correlation between
the strength of the hot wind and the amount of flux in the emission
lines led also to the conclusion that this type of wind provides a
significant contribution to the hydrogen and metal emission
lines. We have also witnessed an exceptional activity during one of the
nights which may be attributed to an increase in LkH
264's
accretion rate or to a flare-like event. Although it is not possible to
clearly distinguish between these possibilities, the available data set
points towards variable accretion as being responsible for the observed event.
Key words: stars: formation - stars: pre-main sequence - stars: individual: WY Arietis
This star also presents a blue excess emission at optical wavelengths and a
Balmer jump (Valenti et al. 1993). The optical continuum of
LkH 264 has been interpreted by Valenti et al. (1993) in the context of a boundary layer model and more
recently, by Costa et al. (1999) as an analogue of the Sun
with a much higher level of activity. By combining UV and optical data
the latter authors find that the continuum could be well fit, from 1200 Å to 7000 Å by free-free and free-bound emission from a hot dense
gas plus blackbody emission at two different temperatures, one related
to the stellar spectral class and the other associated to an hot spot
covering a small fraction of the stellar surface.
Gameiro et al. (1993) present UV and optical spectroscopic
observations of LkH 264. In their work, time series of high
resolution optical spectra for the regions of the H
,
He I
5876 Å and Na I D lines are reported. In a later work Lago &
Gameiro (1998) show time-series analysis of high-resolution
profiles of the same lines. In this case, the authors claim to find
correlations in the behaviour of the equivalent widths of He I and Na I
D which holds for timescales of 1 day down to 1 hour. However, this
correlation apparently breaks down for timescales of
20 min.
In this paper we present optical spectroscopic and photometric
observations of LkH 264 and discuss the observed
variability. In Sect. 2 we describe the observations and the data
reduction. Section 3 is dedicated to the analysis of the spectroscopic
properties of the star in terms of continuum and emission
lines. Differential photometry data are shown in Sect. 4. A detailed
discussion follows in Sect. 5 and we close with general conclusions in
Sect. 6.
The observations discussed here were obtained with the Isaac Newton Telescope (INT), the William Herschel Telescope (WHT) and the Jacobus Kapteyn Telescope (JKT), at the La Palma Observatory. INT spectroscopic observations were carried out from 1993 November 26 to 30 and in 1999 October 21. WHT observations were done during the night of 1999 July 27 with ISIS and during the night of 1999 October 19 with UES. JKT differential photometry was obtained from 1999 September 13 to 18 and from 1999 October 3 to 6.
Date | JD-2 449 000 | band | T![]() |
(sec.) | |||
1993 Nov. 26 | 318.43 | H![]() |
1000 |
1993 Nov. 26 | 318.44 | He I + Na D | 1000 |
1993 Nov. 26 | 318.50 | H![]() |
1000 |
1993 Nov. 26 | 318.51 | He I + Na D | 1000 |
1993 Nov. 26 | 318.53 | He I + Na D | 1000 |
1993 Nov. 26 | 318.54 | H![]() |
1000 |
1993 Nov. 26 | 318.60 | He I + Na D | 1000 |
1993 Nov. 26 | 318.62 | H![]() |
1000 |
1993 Nov. 26 | 318.63 | low | 200 |
1993 Nov. 26 | " | low | 200 |
1993 Nov. 28 | 320.34 | H![]() |
1000 |
1993 Nov. 28 | 320.55 | H![]() |
1000 |
1993 Nov. 28 | 320.56 | He I + Na D | 1000 |
1993 Nov. 28 | 320.36 | low | 200 |
1993 Nov. 29 | 321.36 | low | 200 |
1993 Nov. 29 | 321.51 | He I + Na D | 1000 |
1993 Nov. 29 | 321.60 | H![]() |
1000 |
1993 Nov. 29 | 321.62 | He I + Na D | 1000 |
1993 Nov. 29 | 322.44 | He I + Na D | 1000 |
1993 Nov. 29 | 322.46 | H![]() |
1000 |
1993 Nov. 30 | 322.54 | low | 200 |
1999 Jul. 27 | 2386.5 | blue | 240 |
1999 Jul. 27 | 2386.5 | red1 | 120 |
1999 Jul. 27 | 2386.5 | red2 | 120 |
1999 Oct. 19 | 2470.7 | - | 900 |
1999 Oct. 21 | 2472.5 | low | 1200 |
The INT Intermediate Dispersion Spectrograph (IDS) was used with the
500-mm camera plus a EEV5 4001280 pixels CCD in 1993. Two
gratings, 1800 and 150 lines/mm, were used giving different resolutions
and spectral coverages. As determined from the FWHM of arc spectrum
lines, the resolutions obtained were 0.4 and 4.5 Å respectively. The
higher resolution spectra (grating of 1800 lines/mm) were centred at
5880 Å and 6560 Å and cover a band 240 Å wide. The lower
resolution spectra cover the wavelength range from 3800 Å to 7000 Å.
The ISIS spectrograph at the WHT was used to obtain spectra covering the
wavelength range between 3020 and 8040 Å. This coverage was
achieved by obtaining one spectrum in the ISIS blue arm and two spectra,
centred at different wavelengths, in the ISIS red arm. The blue arm was
setup with the R300B grating and EEV12 (21484200 pixels) CCD,
giving a resolution of 0.86 Å per pixel. The red arm was setup with
the R316R grating and TEK4 (1124
1124 pixels) CCD, providing a
resolution of 0.79 Å per pixel. The three spectra were combined in
order to obtain the desired wavelength coverage.
The UES was used with the E31 grating, set up a central wavelength
of 6548 Å. The slit width of 1.30 arcsec and the length of 6 arcsec,
provided a full spectral coverage between 4663 and 10 265 Å in a
total of 66 echelle orders with
.
The log of the spectroscopic observations is presented in
Table 1. In this table IDS high resolution data is identified
by "H'' or "He I+Na D'' respectively if centred at 6560 Å or
5880 Å, IDS low resolution data is identified by "low'', ISIS data is
identified by "blue'', "red1'' and "red2''.
The spectral reduction was standard including background subtraction,
flat-fielding with a tungsten lamp, removal of cosmic rays and
wavelength calibration. The low resolution spectra obtained in 1993
and in 1999 were flux calibrated with the spectrophotometric standard
star HD19445. The quality of the flux calibration was tested by
comparing the derived values for the most stable stars in the sample
with those found in the literature. This test, complemented by night
to night comparison of flux standards, indicates that uncertainties
in the flux calibration are less than 5% between 4000 and 7000 Å. Another estimate has been done by comparing two
consecutive observations taken with a time interval of 200 s. During this short interval we can assume that the continuum
variation is small and the difference in flux can be used as an estimate
of the flux uncertainty. We obtain differences in the continuum flux
which are less than 3% for wavelengths longer than 5500 Å but that
increase towards the blue, reaching almost 15% for wavelengths
shortward of 4000 Å.
![]() |
Figure 1:
Thin solid lines: LkH![]() ![]() |
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The JKT was used with the JAG-CCD camera coupled to the Site2
(20482048 pixels) detector giving a pixel scale of 0.33
arcsec/pixel. Images were taken in two filters: Sloan g (g') and Sloan r(r'), giving an unvignetted field of view of about 8.5 arcmin
diameter. The log of the JKT observations is presented in Table 2.
Date | JD | Sloan g | Sloan r | ||
-2 449 000 | sec(![]() |
![]() |
sec(![]() |
![]() |
|
(s) | (s) | ||||
1999 Sep. 13 | 2435.63 | 1.061 | 180 | 1.071 | 60 |
1999 Sep. 14 | 2436.66 | 1.022 | 60 | - | - |
1999 Sep. 15 | 2437.73 | 1.046 | 60 | 1.043 | 40 |
1999 Sep. 16 | 2438.74 | 1.072 | 60 | 1.068 | 40 |
1999 Sep. 17 | 2439.68 | 1.075 | 60 | 1.071 | 40 |
1999 Sep. 18 | 2440.74 | 1.073 | 60 | 1.068 | 40 |
1999 Oct. 03 | 2455.75 | - | - | 1.269 | 12 |
1999 Oct. 04 | 2456.73 | - | - | 1.180 | 12 |
1999 Oct. 05 | 2457.74 | 1.204 | 50 | 1.220 | 12 |
1999 Oct. 06 | 2458.73 | 1.243 | 100 | 1.220 | 20 |
The JKT data was reduced following standard IRAF procedures. For each night, flat fielding was achieved from a master flat produced from sky frames obtained either in the evening and/or dawn twilight of that same night. Instrumental magnitudes for the target objects were obtained with the APPHOT package on IRAF, taking an aperture 20 pixels in radius and an annular sky aperture, centred on the star, with inner radius of 25 pixels and 5 pixels wide. The errors in instrumental magnitudes were estimated from a Poissonian noise model which takes into account the detector's read noise and gain.
The general structure of LkH 264's optical spectrum is similar
to those of many other CTTS. The strongest lines present are those from
the Balmer series, Ca II H and K and several other metallic lines
dominated by Fe II, Fe I, He I, Ti II and Na I. Large Fe II blends are seen at
wavelengths centred at 4200, 4600 and 5300 Å, making difficult the
determination of the continuum level there. The weakness of the
absorption lines indicates the presence of continuous and/or line excess
emission.
The observed low resolution spectra (flux calibrated) are shown in Fig. 1 as thin solid lines. With one exception, they cluster at continuum levels below 10-13.5 erg cm-2 s-1 Å-1. The largest continuum variations are observed at shorter wavelengths.
In 1999 we had the chance to observe the star twice, in July and
October. In this case, the continuum level was similar to that observed
during the 1993 run with the exception of the last night of the run
(JD 2449322.54). Also, Valenti et al. (1993) and Mendoza et al. (1990) published flux calibrated spectra of this star,
obtained during October 1981 and August 1989, respectively. The former
extends from the Balmer jump to H
and the latter covers roughly
the same spectral range as our own data. The continuum level in their
spectra occurs at a very similar level to that observed by us in the
first four nights of the 1993 run and in the 1999 spectra. This suggests
that the continuum level in LkH
264 spectra is typically below
10-13.5 erg cm-2 s-1 Å-1 (in the blue), with the
November 30 observation catching the star in an atypical state.
Applying the transmission curve for the V band to the flux calibrated
spectra obtained during this atypical night and the previous one, we
estimate a flux ratio of roughly 2, i.e. a variation in magnitude of
0.75 over one night. As we will show in Sect. 3.2, this
increase correlates with an increase in the veiling, as measured from
our IDS high resolution spectra obtained during the same nights. This
correlation confirms that the flux variation does not result from errors
during the flux calibration for this particular spectrum.
The low resolution spectra presented here allow us to derive the spectrum of the excess flux by assuming the shape and intensity of the stellar photosphere. The shape of the photospheric contribution of a TTS can be represented reasonably well by that of a main sequence star of the same spectral type. In this work, we have used a K5 V spectrum from the stellar spectral flux library of Pickles (1998).
The absolute level of the photospheric contribution was fixed by
equating the flux near 5850 Å, obtained from the calibrated low
resolution spectrum obtained at JD = 2449318.60, to the veiling at that
wavelength , as determined
from the high resolution spectrum obtained at JD = 2449318.63, i.e. data
separated in time by just under 45 minutes. We reddened the
photospheric spectrum using an optical extinction of
AV=0.8 (Costa et al. 1999) and the Cardelli et al. (1989) reddening curve. The photospheric
contribution so determined is overplotted as a solid thick line in
Fig. 1. Spectra of the excess emission, determined by
subtracting the photospheric contribution from the observed spectra, are
shown in Fig. 2.
![]() |
Figure 2: Spectra of the excess flux for the nights of 1993 Nov. 30 (top), 1993 Nov. 26 (middle) and 1993 Nov. 29 (bottom). The remaining excess spectra (not shown) fall between the latter two spectra. The solid thick line represents a 8700 K black body reddened with an AV of 0.8 and covering 6% of the stellar surface (see Sect. 5.1). |
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The IDS high resolution spectra were used to determine the amount of
veiling in LkH 264, i.e. the ratio of the excess flux to the
photospheric flux. The photospheric lines of LkH
264 have been
identified as the veiled photospheric spectrum of a K3 - K5 star and
the veiling was derived using a method based on that described in
Hartigan et al. (1989), which compares the spectrum of the
T Tauri star with that of a template star within a small wavelength band
(a few tens of Ångstr
ms wide) and assuming the veiling to be
constant in this region. We used HD 16160 (K3V) as template star,
rotationally broadened to a
of 20 km s-1, in order to get the
same blend of lines as in LkH
264 spectrum. The veiling was
computed in the regions 5910-6000 Å and 6600-6670 Å, near the
He I+Na D and H
emission lines respectively. The main photospheric
lines in these regions are Fe I lines.
The night to night variations in these two bands are quite similar
(Fig. 3). We should note that in comparison with
veiling determinations in the He I+Na D band, the uncertainties in the
veiling near H
are significantly larger (less than 10% in the
former and at times larger than 25% in the latter). This is due to the
intrinsic weakness of the photospheric lines of a K3-5 dwarf near
H
,
and also due to a larger uncertainty in the placement of the
continuum level in this band. The latter results from the presence of
very extended wings in the H
emission line.
![]() |
Figure 3:
The veiling level during 1993 Nov. 26-30 in the
bands near the H![]() |
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During the night of 1993 Nov. 26 (JD 2449318), we obtained four spectra in both
bands and it is clear that the measured absolute veiling shows strong
variations, the veiling varies from 0.3 to 0.5 on a time scale of
hours. These variations on relatively short timescale seem to be
characteristic of this star. Furthermore, during the night of 1993 Nov. 28 (JD
2449320), the veiling around the H
region decreased by a
factor of 3 in just 5 hours, suggesting large
variations of the amount of excess emission on relatively short time
scales.
The quasi-simultaneous observations in high and low resolution during
the 1993 run allow to compare the veiling variability, as determined
from the IDS high resolution spectra, with the continuum variability, as
determined from the low resolution spectra. If the veiling was just an
effect of continuum excess, we would expect a strong correlation between
veiling and continuum flux. Since the veiling is a relative measure of
the continuum excess (non-photospheric) flux, it cannot be directly
compared to the observed continuum flux at a given wavelength. However,
it is easily seen that the ratio of the observed continuum flux at a
given wavelength, in two different nights i and j,
,
is equal to the ratio
,
where
and
are the veilings in nights i and j at wavelength
.
In Fig. 4 we compare the observed continuum flux (from low resolution data) to the observed veiling (from veiling determinations). Comparisons are done in the sense described in the paragraph above with all ratios computed with respect to JD = 2449318.60 and JD = 2449318.63, respectively for continuum flux ratios and veiling ratios. Clearly the veiling and continuum ratios coincide within the error bars. This shows that indeed, the observed veiling results from an extra source of continuum, not to emission in the lines themselves.
![]() |
Figure 4:
Veiling ratios in the bands near the H![]() ![]() |
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The veiling was also computed from the echelle spectrum obtained on the night JD 2451470.7. In this particular spectrum it is possible to determine the veiling variation with wavelength from roughly 5200 to 8500 Å. The lower S/N at the region blueward 5000 Å and the weakness of photospheric lines for wavelengths larger than 8500 Å prevent any veiling determination in these regions. The results are plotted in Fig. 5.
![]() |
Figure 5: Veiling variation with wavelength for JD 2451470.7 (from nine echelle orders). |
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The steep rise in veiling towards the blue and a
smooth increase redwards 6700 Å is quite clear. The veiling near 5870 Å, i.e.,
near the He I and Na D lines, is
,
and near H
line
is
in good agreement with the values found at those
regions during the period between JD 2449318 and 2449321. This fact is
an indication that the continuum level at the time of the echelle
observation (JD 2451470.7) was similar to the average continuum level
observed during the 1993 run and also similar to that observed at low
resolution two nights later in JD 2451472.5.
In Table 3 we list the fluxes of the strongest emission
lines observed in the low resolution spectra. The fluxes were obtained
for each line by multiplying its equivalent width by the mean flux level
of its neighbouring continuum.
It should be noted that the pairs CaII H + H
and He I + Na D are not fully resolved. For those lines
Table 3 gives only the combined fluxes. Figure 6
displays the temporal variations of line fluxes. Due to the large time
gap between the 1993 and 1999 observations, the x-axis in these figures
is non-continuous. The uncertainties in fluxes are directly related to
the uncertainties regarding the continuum placement and accuracy of flux
calibration (see Sect. 3.1).
JD | 2449318.63 | 2449320.36 | 2449321.36 | 2449322.54 | 2451386.5 | 2451472.5 |
H8 | 1.02e-13 | 1.85e-13 | 1.06e-13 | 1.80e-13 | 8.1e-14 | 3.6e-14 |
Ca II K | 8.64e-13 | 8.84e-13 | 6.59e-13 | 1.30e-12 | 6.50e-13 | 3.22e-13 |
Ca II H | 6.47e-13 | 6.32e-13 | 4.71e-13 | 7.93e-13 | 5.5e-13 | 2.6e-13 |
H![]() |
2.32e-13 | 2.86e-13 | 1.96e-13 | 3.21e-13 | 1.67e-13 | 5.7e-14 |
H![]() |
3.49e-13 | 3.84e-13 | 3.e-13 | 4.81e-13 | 2.34e-13 | 1.26e-13 |
H![]() |
8.10e-13 | 9.67e-13 | 7.17e-13 | 1.24e-12 | 6.33e-13 | 2.59e-13 |
Fe II (![]() |
1.19e-13 | 1.46e-13 | 9.5e-14 | 1.42e-13 | 7.7e-14 | 1.9e-14 |
Fe II (![]() |
1.45e-13 | 1.79e-13 | 1.07e-13 | 1.73e-13 | 8.8e-14 | 1.9e-14 |
Fe II (![]() |
1.69e-13 | 2.11e-13 | 1.48e-13 | 2.31e-13 | 1.12e-13 | 2.3e-14 |
Fe II (![]() |
9.8e-14 | 1.32e-13 | 8.6e-14 | 1.37e-13 | 6.5e-14 | 4.0e-14 |
He I + Na D | 1.15e-13 | 1.33e-13 | 9.3e-14 | 1.30e-13 | 5.1e-14 | 2.6e-14 |
H![]() |
2.92e-12 | 3.88e-12 | 2.98e-12 | 5.51e-12 | 3.29e-12 | - |
![]() |
Figure 6: Flux of the strongest emission lines observed in the low resolution spectra. |
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In order to compare the variations in the line fluxes with the variations in the continuum level we plot in Fig. 7 the continuum level near each of the emission lines. The dashed line in Fig. 7 represents the mean continuum level obtained by excluding the data point corresponding to JD 2449322.54, the atypically high observed continuum level. The continuum levels measured form the spectra published by Mendoza et al. (1990) and Valenti et al. (1993) are shown in Fig. 7 as solid and dotted lines, respectively. These should be regarded as rough estimates only.
![]() |
Figure 7: Continuum level near emission lines observed in the low resolution spectra. |
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As expected, Fig. 7 shows that the continuum level near the selected lines is roughly constant, with the exception of that corresponding to the spectrum observed at JD 2449322.5. Relative emission line flux variations, with respect to the mean, are larger than those observed in the continuum, with a clear decrease in flux for all the lines in the last spectrum (JD 2451472.5). In Table 4 we list the correlation coefficient of lines and lines versus their nearby continuum. It should be noted that the small number of points taken in this calculation does not allow a high confidence level. In Table 4 we represent in bold the coefficient for which the false alarm probability is less than 0.01. The numbers suggest that, in general, the behaviour of the emission lines is well correlated.
In contrast to the correlation found in the behaviour of the emission lines amongst themselves, emission lines and continuum flux do not seem to be in general correlated. While the maximum observed flux of almost all lines occurs on the same night as the maximum continuum level, i.e. at JD 2449322.54, when we consider all nights we see no significant correlation between emission line and continuum flux (see Table 4). Hence, variations in line emission appear to be, often, decoupled from variations in the continuum.
In the particular case of JD 2449322.54, the relatively large increase in the continuum level was accompanied by an increase in the line fluxes. Although relative variations differ from line to line the observations suggest that the source producing the increase in the continuum level during this night also produced the strengthening of the emission lines.
H8 | CaIIK | CaIIH | H![]() |
H![]() |
H![]() |
Fe II | Fe II | Fe II | Fe II | He I | H![]() |
continuum | |
![]() |
![]() |
![]() |
![]() |
||||||||||
H8 | 1.00 | 0.87 | 0.83 | 0.95 | 0.92 | 0.93 | 0.94 | 0.94 | 0.95 | 0.98 | 0.91 | 0.77 | 0.31 |
CaIIK | 0.87 | 1.00 | 0.97 | 0.95 | 0.97 | 0.98 | 0.89 | 0.89 | 0.93 | 0.91 | 0.86 | 0.91 | 0.61 |
CaIIH | 0.83 | 0.97 | 1.00 | 0.95 | 0.94 | 0.96 | 0.92 | 0.92 | 0.93 | 0.88 | 0.85 | 0.83 | 0.51 |
H![]() |
0.95 | 0.95 | 0.95 | 1.00 | 0.99 | 0.99 | 0.99 | 0.98 | 1.00 | 0.98 | 0.95 | 0.82 | 0.28 |
H![]() |
0.92 | 0.97 | 0.94 | 0.99 | 1.00 | 0.99 | 0.95 | 0.95 | 0.98 | 0.97 | 0.95 | 0.82 | 0.50 |
H![]() |
0.93 | 0.98 | 0.96 | 0.99 | 0.99 | 1.00 | 0.95 | 0.94 | 0.98 | 0.96 | 0.91 | 0.92 | 0.63 |
Fe II | 0.94 | 0.89 | 0.92 | 0.99 | 0.95 | 0.95 | 1.00 | 1.00 | 0.99 | 0.97 | 0.97 | 0.63 | 0.41 |
Fe II | 0.94 | 0.89 | 0.92 | 0.98 | 0.95 | 0.94 | 1.00 | 1.00 | 0.99 | 0.97 | 0.98 | 0.63 | 0.39 |
Fe II | 0.95 | 0.93 | 0.93 | 1.00 | 0.98 | 0.98 | 0.99 | 0.99 | 1.00 | 0.98 | 0.97 | 0.77 | 0.52 |
Fe II | 0.98 | 0.91 | 0.88 | 0.98 | 0.97 | 0.96 | 0.97 | 0.97 | 0.98 | 1.00 | 0.97 | 0.75 | 0.54 |
He I | 0.91 | 0.86 | 0.85 | 0.95 | 0.95 | 0.91 | 0.97 | 0.98 | 0.97 | 0.97 | 1.00 | 0.51 | 0.40 |
H![]() |
0.77 | 0.91 | 0.83 | 0.82 | 0.82 | 0.92 | 0.63 | 0.63 | 0.77 | 0.75 | 0.51 | 1.00 | 0.98 |
Amongst the emission lines seen in the spectrum, the He I lines are particularly interesting, due to their high excitation potential. The lines can be formed in two ways: collisional excitation, requiring gas temperatures larger than 25 000 K, or photoionization followed by recombination and cascade, requiring local kinetic temperatures between 8 000 and 15 000 K (see Beristain et al. 2001 - hereafter BEK01- and references therein).
LkH
264 shows He I
Å and He I
Å in emission. In addition, both lines are variable in the shape of
their profiles and in their strength. Figure 8 illustrates
this fact for the He I
Å line, where the line profiles
observed on October 19 1999 and on November 28 1993 are shown. The He I
Å profiles (not shown) are similar in shape to those
of He I
Å yet narrower and less intense. See Fig. 7
of Lago & Gameiro (1998) for the full set of He I
Å profiles obtained during the 1993 run.
![]() |
Figure 8:
He I
![]() |
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![]() |
Figure 9:
Centre velocity of the He I lines narrow (NC) and broad (BC)
components. Top panel: He I
![]() ![]() |
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According to several authors, the He I line profiles in CTTS are formed by two components (e.g. Hamann & Persson 1992; Batalha et al. 1996). A narrow component (NC), of Full Width Half Maximum (FWHM) typically smaller than 2 Å, whose origin is usually attributed to the post-shock region of an accretion shock and a broad component (BC), with typical FWHM up to around 6 Å, which is believed to contribution to line emission from a hot wind and from accreting material (see BEK01).
Lago & Gameiro (1998) suggest that for LkH
264 the
He I
Å profile is made up of two components but did
not analyse each of them quantitatively. Following the work of BEK01
we decomposed the He I
Å and
Å lines
into broad and narrow components by Gaussian fitting.
The BC and NC in LkH 264 are characterised by FWHM of
Å and
Å respectively. Figure 9 shows the centre velocity of each component for both
lines. Uncertainties in the computation of these velocities are smaller
than
,
which is the spectral resolution of the
data.
The NCs in LkH
264's He I profiles peak near the stellar rest
velocity, with no significant shift between the He I
and
He I
NCs. In contrast, the BCs of the He I lines tend to
be slightly blueshifted, between 50 and
for He I
and up to about
for
He I
.
An exception to this behaviour occurs in the night
JD 2449322.54, when the BCs of both lines peak near zero velocity (see
Fig. 9). Hence, the state of high continuum level
observed during that night is accompanied by a clear shift towards the
red of the BCs of the He I profiles.
He I
to He I
line ratios give an indication
of the physical conditions where they are formed. At low densities the
triplet to singlet line ratio is around 3.5. Initially, as density
increases so does this ratio. However, at high enough densities the
line ratio starts to decrease, approaching values near unity (see BEK01
and references therein). We determined line fluxes in the He I lines by
combining the IDS high resolution data, from which we computed the
equivalent widths of the narrow and broad components, with the low
resolution flux-calibrated spectra, from which we determined the
continuum flux in the vicinity of the lines. Results are shown in
Table 5. Uncertainties in the line ratios were estimated
from the uncertainties in the determination of the continuum level near
the emission line, from the low resolution data and from the formal
uncertainties that result to the equivalent widths from Gaussian
fitting.
JD | 2449318.6 | 2449320.4 | 2449321.4 | 2449322.5 |
NC | 1.8 ![]() |
3.7 ![]() |
2.7 ![]() |
3.2 ![]() |
BC | 4.3 ![]() |
2.5 ![]() |
3.6 ![]() |
4.2 ![]() |
In general, the line ratios are larger for BC than for NC, as expected
if the region in which the BC originates is of lower density than that
where the NC arises. However, with the exception of JD 2449318.6,
the NC ratios are significantly higher than the average in BEK01. This
indicates that, during the period of time discussed here, densities in
LkH
264's NC formation region are generally lower than those in
the corresponding region of the average T Tauri star.
The high resolution echelle spectrum taken on the night of October
19 1999 allowed us to obtain line profiles which contain kinematic
information that can be used to understand mass flows in the stellar
atmosphere. We find spectroscopic evidence for the presence of stellar
winds from blueshifted absorption in several emission lines. Inverse P
Cygni profiles in two lines, O I
and O I
(Fig. 10) reveal the presence of infalling material. The O I
line profile shown here is the residual normalised line
profile, i.e. the photospheric contribution has been removed by
subtracting a K3 V template. The O I
line profile did not
have the photospheric contribution removed since we do not have the
spectrum of the template around that wavelength region. The absorption
features observed near
,
and
are photospheric in origin,
corresponding respectively to TiI lines at 8426.5, 8434.9 and
8435.6 Å and to the 8468.4 Å Fe I line.
The centre of the redshifted absorption features lie near
,
while their widths are about
at the continuum level. The absorptions meet the continuum in
the red slightly above
.
These velocities are
consistent with infall from a height of a few stellar radii for typical
T Tauri stars' parameters (Edwards et al. 1994). These line
profiles indicate that infall was occurring when the spectra was
acquired.
The observed field containing LkH 264 was chosen so as to also
include stars bright enough to be used as reference for differential
photometry. The chosen field contains five such stars (henceforth
referred to as field stars), about which not much information exist
other than their approximate magnitudes from the USNO Catalogue (as
consulted at ESO with GAIA). After examining the differential
photometry of the various field stars against each other we decided to
choose three of them to serve as reference for LkH
264. A
virtual reference star was constructed for each night by averaging the
instrumental magnitudes of the three chosen field stars.
LkH
264's differential photometry was computed relative to the
virtual reference star. Results are shown in Fig. 11
where independent data points are displayed between 0 and
.
An estimate of the uncertainty in each data point is given by the
standard deviation of the nightly sequence of differential magnitudes of
the three field stars, which amounts to 0.01 at g' and 0.02 at r'.
These uncertainties, smaller than the symbol size in Fig. 11, dominate those that result from Poissonian statistics
in determining the instrumental magnitudes.
![]() |
Figure 10:
Top panel: residual normalised O I
![]() ![]() |
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LkH 264 show variability in both g'- and r'-bands. As commonly
observed in T Tauri stars, the amplitude of the variation is higher in
the blue than in the red. At the g'-band, the star's brightness
varies by about 0.6 mag between maximum and minimum intensity
whereas the same variation at the r'-band is about 0.2 mag.
Variations in both bands seem to occur nearly in phase, with no hint
for a significant relative shift in time between the g'- and r'-band
light curves.
![]() |
Figure 11: Differential photometry. Differential magnitudes were normalised to zero by subtracting their average value. The lines represent best fit periodical solutions to the data (period 3.04 days - see Sect. 5.1). The data is represented in a phase diagram corresponding to the above period. |
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The data presented here allow us to identify different types of
variability in LkH 264: variations in the continuum level
characterised by different amplitudes, variations in the continuum
dependency with wavelength, variations in emission line fluxes and line
profile variability. The timescales in which such variations occur also
seem to be diverse.
Variability in TTS is usually explained by rotational modulation by cool or hot spots, by variable accretion, by chromospheric activity or by inhomogeneous circumstellar envelopes (e.g. Appenzeller & Mundt 1989).
Inhomogeneous circumstellar envelopes are unlikely to be the cause for most of the observed variations in the continuum flux described here. Extinction would decrease and redden the stellar flux but it would produce no veiling effect. The correlation found between the variations in the observed veiling and variations in the observed continuum flux (Fig. 4), is a strong indication that the cause of continuum variability is an extra continuum source that veils the stellar photospheric spectrum.
Rotational modulation of LkH 264 brightness is far from being
well established. Mendoza et al. (1990) performed
Strömgren photometry for this star during eleven consecutive nights in
October/November 1986. Their study shows no signs of periodic
behaviour. Fernandez & Eiroa (1996) suggest a rotation
period near 2.6 days (August 1991 data), but their photometric data
suffer from inadequate time sampling. The differential photometry
presented here suffers from the same problem. We tentatively fit a
simple periodic solution to our observations. A Lomb Periodogram
analysis yields a period of 3.04 days but with very modest false alarm
probabilities, respectively 0.42 and 0.65 for g' and r'. Given the above
period, a linear least squares fit for the amplitudes and phases was
performed. We found amplitudes of 0.22 at g' and 0.09 at r' with the
variations occurring very nearly in phase in the two wavebands. We plot
that solution in the phase diagram in Fig. 11. We note
that such a period is consistent with that of the periodical variations
found by Gameiro et al. (1993) in the behaviour of the He I
and Na D emission lines. A better time sampling is necessary if such a
period is to be confirmed.
Periodic variations in TTS are interpreted as being due to the presence of hot or cool spots in the star's surface. Fernandez & Eiroa (1996) model their photometric observations with a hot spot with temperatures in the range 4600-6400 K and covering factors between approximately 37% and 2%. While we cannot make a similar analysis with our photometric data (we only have differential photometry for g' and r'), the fact that the amplitude of the variations in the g'-band is roughly double that observed in the r'-band hints that if they are due to the presence of a spot it is most likely a hot spot.
In Costa et al. (1999) the observed UV spectrum and the JD
2449322.54 optical spectrum of LkH 264 are fitted with the sum
of three components: a stellar black-body at 4300 K, hydrogen
free-free plus free-bound emission at
K (that dominates
continuum emission only in the UV at wavelengths shorter than 2000 Å)
and an extra black-body at 8700 K covering 4% of the stellar surface,
all subject to a Savage & Mathis (1979) extinction law
with AV=0.8. The latter black-body component can be
interpreted as a hot spot on the star's surface. If we replace the above
stellar black-body contribution by the photospheric contribution
discussed in Sect. 5.1, we explain the excess emission of the JD
2449322.54 spectrum by adding a contribution from a 8700 K black-body
covering 6% of the stellar surface (see Fig. 2), in tune
with the results from Paper I. The excess continuum emission from the
remaining nights could not be fitted by black-body emission. The excess
spectrum beyond around 5500 Å falls much faster than a black-body
does. By assuming a very low extinction (AV<0.1) one can
reproduce the slopes observed in the excess spectra of those
nights. However, decreasing the extinction to such values would lead to
a substantial increase in the observed flux, which goes against the
observations: the steepest excess spectra correspond to the fainter
states of LkH
264.
To compare the variations observed in the low resolution spectroscopy
with those observed photometrically we performed g' and r' synthetic
photometry on the flux calibrated spectra. The results show that
variations during the November 1993 run are within 0.2 mag for g' and
0.1 mag for r', except for the night of JD 2449322.54, when
LkH 264 was 0.61 mag brighter than average in g' and 0.57 mag
brighter than average in r'. Hence, the November 1993 variations seem
to be similar to those observed during 1999, with the exception of that
of JD 2449322.54, which probably corresponds to an exceptional event.
Hot spots in T Tauri stars are usually thought to result from a shock,
as accretion flows hit the stellar photosphere. Evidence for accretion
in LkH 264 has never been clearly established from a
dynamical point of view. While one could argue that the observed
profiles of H
and H
display characteristics that are
typical of those produced by magnetospheric accretion models, such as
being centrally peaked, slightly blueshifted and with blue to red
asymmetry factors slightly larger than one (Hartmann et al.
1994; Edwards et al. 1994; Muzerolle et al. 1998), unequivocal model-independent
evidence for accretion comes only from the presence in the line profiles
of redshifted absorption falling bellow the continuum. We could find no
record of emission lines in spectra of LkH
264 with such a
characteristic until now. The O I line profiles we present here seem
to be the first clear model-independent indication that LkH
264
is actively accreting (recall Fig. 10).
When the O I observations were obtained (19 October 1999) the star did
not seem to be in a particularly active state. For that date, the
amount of veiling computed at 5870 Å is
.
Such a value
corresponds to the typical amount of excess emission found in
LkH
264, not to the exceptionally high state observed on JD
2449322.54 (see Fig. 3). In addition, the He I 5876 Å and the He I 6678 Å lines observed then, lack the broad
component clearly identified during the 1993 campaign (see
Fig. 8 for a comparison between the 1999 He I 5876 Å
profile and a corresponding 1993 profile).
BEK01 analyse the helium emission from a sample of 31 CTTS and propose a
dual origin for the broad component: magnetospheric infall and/or a hot
wind. BC line centres occurring at redshifted velocities larger than
are indicative of origin in funnel accretion flow,
whereas BC with line centres blueshifted by more than
likely arise in a hot wind. Furthermore, they suggest that when
a hot wind is present, the luminosity and temperature of the accretion
shock seems to decrease. Our Lkh
264 observations reveal that
when the continuum level is around its average value (lowish excess
emission) the BC in the He I
is blueshifted by more than
.
On the other hand, when the continuum level
is high (larger excess emission), the He I
BC is slightly
redshifted or centred very near the star's rest velocity (recall Fig. 9). The shift towards the red of the He I lines is
accompanied by a similar shift in the Na D lines. The latter were
blueshifted by nearly
in November 26 and
redshifted by nearly
in November 30. These
similar shifts suggest that both lines could be formed in a common
region. Such an idea had already been suggested by Lago & Gameiro
(1998), as the result of the strong correlation between Na D
and He I
equivalent widths observed during two runs in
1990 and 1993. The main contributor for the total equivalent width of
the He I lines is the BC. The He I vs. Na D correlation we find is in fact
between the He I
BC and Na D lines (we found a linear
correlation of 0.83). There is no clear correlation with the narrow
component (linear correlation of 0.48). A similar result is found when
comparing the behaviour of both components of He I
with Na D, although in this case the correlation is not as high (we determined a
correlation of 0.70 for the broad component and no correlation with the
narrow component).
In addition, the presence of a wind during the nights of 1993 November
28 and 29 is revealed by the Na D and H
line profiles (see
Figs. 3 and 7 in Lago & Gameiro 1998) which, for those
nights, display a narrow absorption blueshifted by about
.
During the November 1993 run, the hot wind seemed stronger on the night of the 28th (largest equivalent width of the He I blueshifted broad component). Emission line fluxes observed that night are, in general, larger than those observed in the remaining "quiet'' nights. On the other hand, in October 1999, when we see no BC in the He I lines, all emission lines are relatively faint. This correlation between the strength/absence of a hot wind and the amount of flux in the emission lines seems to indicate that a hot wind provides a significant contribution towards both hydrogenic and metallic line emission.
In the BEK01 picture, the lack of a BC on the October 1999 profile
indicates that, at that particular date, a hot wind is not present
nor is the accretion flow hot enough to produce emission in the He I
lines. Despite the lack of this signature for a hot wind, a cooler wind
(cooler than that traced by He I in emission) still seems to be present
in Lkh 264. The H
,
H
and infrared CaII triplet
line profiles all provide indications for such wind (see
Fig. 12). In all these lines there is a clear asymmetry in
the blue wing of the emission profile, between -100 and
,
indicative of outflowing material.
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Figure 12:
19 October 1999 H![]() ![]() |
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Apart from the blueshifted asymmetries, all five line profiles also
display asymmetries redward of the line centre. These asymmetries are
very probably related to the accretion flow, so clearly identified in
O I, showing that these lines have contributions from distinct regions
around LkH 264.
Are the changes observed in the line profiles discussed above related to the variability in the continuum emission?
The anomously high continuum level observed on JD 2449322.54 is definitely accompanied by a shift towards the red of the He I BCs. If the shift is the result of changing the dominant source of He I emission from the wind to the accretion flow, as suggested by the BEK01 scenario, the increase in continuum brightness is probably associated with an increase in the accretion rate. On the other hand, when the continuum flux is minimum (JD 2449321.36), the BC of the He I lines is blueshifted and has its largest equivalent width. In terms of the BEK01 scenario, when a hot wind is present the star seems to show a lower continuum brightness.
The variations in continuum and line fluxes show two types of behaviour (compare Figs. 6 and 7). At times, variations in the line fluxes seem directly correlated to variations in the continuum intensity, e.g. the night of November 30 when both continuum and lines increase significantly in brightness. However, an increase in the line fluxes can also occur together with a decrease in continuum intensity, as observed for the night of November 28, or a substantial decrease in line fluxes occurring when the continuum stays roughly constant (e.g. October 1999 observations). Variations in line emission seem to occur a lot more easily than variations in the continuum.
The flux variations of lines is strongly correlated even when we compare
lines with very different excitation potentials such as the Fe II (3 eV) and the hydrogen Balmer lines (
10-13 eV), with only a
few exception, such as H
(Table 4).
Optical flares have been reported both in WTTS and in CTTS (e.g. Gham
1990; Guenther & Ball 1999). They are
characterised by a rapid increase (typically <1 hour) and a slower
decay (up to a few hours from maximum to half-peak luminosity) of their
brightness and H
and H
line emission. The exceptional
increase in brightness observed on JD 2449322.54 could, in principle, be
due to a flare like event. Our data lacks the time sampling resolution
to evaluate such hypothesis. By analysing the change in the continuum
shape that accompanied the event and the variation in line fluxes alone,
we are not able to conclusively distinguish between a flare and enhanced
accretion. However, we note that although the H
and H
emission line fluxes increase from the November 29 to November 30 by a
factor of
1.6, the continuum flux and veiling also increase
between these two nights. We recall that optical veiling is generally
interpreted as the result of excess emission from an accretion shock.
The flare event observed in the CTTS FN Tau by Guenther & Ball
(1999) is characterised by a clear increase in the H
and H
EWs while the veiling and continuum flux remain constant,
hinting that the JD 2449322.54 event might not be associated with a
flare but with variable accretion.
The observations discussed here mostly pertain to line emission and the spectrum of the derived excess emission. These can be directly compared with model predictions for line profiles (Muzerolle et al. 2001) within the now widely accepted magnetospheric accretion paradigm, and with predictions for the continuum excess emission from studies of the structure and emission of the accretion shock in T Tauri stars CG98.
Muzerolle et al. (2001) produced a grid of line
profiles for H,
H
,
H
,
Pa
,
Br
and
Na D1 and D2 covering four accretion rates
(log
/yr)=-6, -7, -8 and -9), five maximum values for
the temperature distribution (6, 7, 8, 9 and 12 thousand K), four
inclination angles (
,
,
and
)
and four
magnetospheric sizes (small and wide, small and narrow, large and wide
and large and narrow)
. The model results for H
and H
were compared with
the observed profiles obtained on 19 October 1999, i.e. those in Fig. 12. A reasonably good match between model and observed
H
profiles, as judged by eye, results only from two sets of
parameters: log
/yr)=-6,
K,
in a small wide magnetosphere (hereafter set 1) and
log
/yr)=-7,
K,
again in
small wide magnetosphere (hereafter set 2). However, when the H
model profiles that result from these two parameter sets are compared
with the observed one, the discrepancy is clear. Both model H
profiles are too wide and too faint. We note that H
and H
were observed simultaneously in a single echelle spectrum. Searching
through all the H
profiles in the model grid reveals that
qualitatively many model profiles are similar to that observed. However,
a quantitative comparison shows clear differences, both in the width and
in the strength of the qualitatively similar line profiles.
In addition to H
and H
profiles, the echelle spectrum
shows that at that date there is no significant emission from the Na D1
and D2 lines (observed Na D1 and D2 not shown here). What are the
predicted Na D1 and D2 profiles for parameter sets 1 and 2? Set 1
predicts profiles with a peak intensity (normalized to the continuum)
above 3.5. Set 2 predicts profiles with a peak slightly above 1.3, in
the same units, and a clear redshifted absorption feature. Neither of
these resemble the observations.
The lack of agreement between the model and the observed line profiles
hints that either LkH 264 has an accretion geometry and/or
physical conditions in its accretion flow that depart significantly from
the treatment used by Muzerolle et al. (2001), or that
the accretion flow is not the only contributor to line emission.
CG98 compute the emergent flux from accretion column models in the context of T Tauri stars. The emergent flux resulting from the models presented in that work (their Fig. 4) can be readily compared to the excess emission flux presented in our Fig. 2. In order to better compare the model predictions with our observations we digitized the solid lines of CG98's Fig. 4.
The shape of the stronger excess emission spectrum (uppermost spectrum
in Fig. 2) can be successfully matched, as judged by eye, by the
log
model reddened by AV between 0.2 and
0.9 or by the log
model with AV=0. CG98
show that the dependence of the model results in the mass over radius
ratio is weak, hence we do not consider such variations here. The
amount of reddening necessary for a match between the
log
and the observations is in general agreement with
the AVs found in the literature for LkH
264
(AV=0.5 in Cohen & Kuhi 1979 and
AV=0.8 in Costa et al. 1999 and Luhman
2001). The AV=0 necessary for a match of the
model with log
is probably unrealistic. If one uses a
stellar radius of
,
typical of T Tauri stars, and a
distance of 65 pc (Hobbs 1986) the above models predict the
observed amount of excess emission for filling factors of around
0.4%. This is in good agreement with the filling factors found by CG98
for the less veiled T Tauri stars in their studied sample, but roughly a
factor of 10 smaller than that that results from the black body
interpretation alluded to in Sect. 5.1. If instead we use
275 pc (Luhman 2001) the filling factor becomes around 6%,
comparable to that derived by the black-body approach.
We were unable to achieve a reasonable match between the CG98 models and
the shape of the excess emission for the lower excess emission states of
LkH 264 (lower two spectra in Fig. 2 and veiling in Fig. 5). The shape of the model predictions is too flat when compared to the
observations, even for AV=0. Increasing the amount of
extinction affecting the predicted model excess makes the problem worse,
since it flattens the shape of the spectrum even further. The sharp drop
in continuum emission, occurring just before 5500 Å, observed for the
lower excess emission states (Fig. 2), is the cause for the model
mismatch. Interestingly, we note that the presence of a similar drop in
the excess emission continuum of the stars modelled by CG98 would not be
apparent, since the wavelength coverage in their observed spectra stops
at 5300 Å.
The veiling measurements displayed in Fig. 5 show the same sort of decrease in excess emission. Comparing the observed veiling with that predicted by CG98 (their Fig. 11) shows, as expected from the considerations above, model veilings decreasing too slowly (we corrected the predicted veiling from a K7-M0 to a K5 photosphere). In addition, the veiling resulting from the models decrease monotonically all the way to the near infrared (NIR) part of the spectrum. Figure 5 shows an increase in the amount of veiling in the very red end of the spectrum, in line with the results obtained by Folha & Emerson (1999), who find higher than expected NIR veiling.
Our comparison between model and observed excess emission spectra is limited to the results shown in Fig. 4 of CG98. A full exploration of the model parameter space would be desirable in order to understand whether continuum excess emission from accretion funnels is capable of explaining the observations discussed here, and in particular the change in continuum slope observed to occur near 5500 Å.
The hypothesis that the origin of the observed variability in
LkH 264 is extrinsic is not supported by the data presented
here. Variations with causes such as variable circumstellar extinction
would not produce a tight correlation between observed intensity and
measured veiling.
Typical variations can be produced by the presence of inhomogeneities on the stellar surface and rotational modulation. If that is the case, unless the lifetime of such inhomogeneities is very small (one or two days), a photometric period still needs to be unambiguously identified.
O I line profiles reveal the presence of an accretion flow onto the star. Such flow would produce one or more hot spots as infalling material shocks near the stellar surface.
The November 1993 He I profiles indicate the presence of a hot wind at
that time. The October 99 data show no signs for the presence of a hot
wind (no BC in He I emission lines). However a cooler wind at the latter
date is revealed by Balmer and CaII infrared triplet line
profiles. Emission lines fluxes in October 1999 are considerably smaller
than those observed in November 1993, despite the identical continuum
intensity. A hot wind seems to contribute significantly to emission in
both hydrogen and metal lines. The lack of success in
explaining the H,
H
and Na D lines within the context of
magnetospheric accretion, reinforces the idea that other line emission
sources are at play on LkH
264.
The exceptional activity registered during the night JD 2449322.54 may
correspond to an increase in LkH 264's accretion rate or to a
flare-like event. Distinguishing between the two interpretations seems
impossible given the available data. A better time sampling during the
event would have to have happened in order to clearly distinguish
between these two possibilities. Nevertheless, the discussion in
Sect. 5 hints for variable accretion as the cause of
the observed changes.
Accretion models are successful in explaining the continuum excess emission observed during the night of JD 2449322.54 but not the continuum slope observed during the other nights for which we have observations. These conclusions are based on the published model results only. A more complete exploration of the model parameter space is necessary to assess this matter fully.
LkH 264 is a CTTS displaying different types of activity. The
presence of infalling matter, hot and cool outflows and the possibility
of enhanced solar like activity such as flares, make it an excellent
target for studying how these different phenomena are related. Such
knowledge is essential if one is to better understand T Tauri stellar
systems, i.e. if one is to better understand how the young Sun
behaved. Synoptic observations on timescales ranging from minutes to
hours to days are the only way to achieve that goal.
Acknowledgements
We thank Dr. Nicholas Walton for donating about 10 min per night during his JKT Supernovae programme to the photometric observations reported here. We thank Javier Mendez and Guillaume Blanc for executing those observations. We also thank an anonymous referee for valuable comments that helped to improve this paper. D. F. M. Folha aknowledges financial support from the "Subprograma Ciência e Tecnologia doQuadro Comunitário de Apoio''. This work is supported by Fundação para a Ciência e Tecnologia, under project POCTI/1999/Fis/34549.