Astronomy & Astrophysics

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$\begin{figure} \par\includegraphics[angle=90,width=18cm,clip]{0070fig1.ps} \end{figure}$ Figure 1: Chandra observation of LkH $\alpha$ 312 on October 18, 2000. The background image is the ACIS-I observation in the energy range 0.5-8 keV of LkH $\alpha$ 312, located at $\sim$ $8\hbox {$^\prime $ }$ off-axis (the pixel size is 0 $.\!\!^{\prime\prime}$ 25). The color scale is stretched logarithmically to enhance the point spread function (PSF) features, note the $2\hbox {$^{\prime \prime }$ }$ asymmetric cusp surrounded by a $10\hbox {$^{\prime \prime }$ }$ elliptical halo. The contour map displays arbitrary intensities from the 2MASS J-band image using linear scale. The positions of X-ray sources detected above the threshold of $3\times 10^{-7}$ are marked with " $\times$ ''. Their infrared positions in the 2MASS survey are marked by "+''.
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In the text

$\begin{figure} \par\includegraphics[angle=90,width=8.6cm,clip]{0070fig2.ps} \end{figure}$ Figure 2: ROSAT/HRI X-ray light curve of LkH $\alpha$ 312 obtained from September 5 to October 5, 1997. Crosses show the X-ray light curve and one-sigma error bars in the energy range 0.1-2.4 keV for each observational segment ( $\sim$ 20 min). Background count rates are shown as upper limits when the background subtracted count rate is negative.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{0070fig3.ps} \end{figure}$ Figure 3: J-H, $H-K_{\rm S}$ diagram of LkH $\alpha$ 312 (open dot). Magnitude error bars are the total magnitude uncertainties at the 90% confidence level. The intrinsic colors of giants (dotted line) and A0-M 6 dwarfs (continuous line) from Bessel & Brett (1988), adapted for the 2MASS photometric system using the 2MASS color transformations (Carpenter 2001; Cutri et al. 2003), are plotted for comparison. The extinction law of Cohen et al. (1981), also adapted for the 2MASS photometric system, is drawn for giants and M 9 stars. The arrow indicates a visual extinction of 1 mag. Black dots mark the position of known classical T Tauri stars in the vicinity of LkH $\alpha$ 312 (Herbig & Bell 1988).
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{0070fig4.ps} \end{figure}$ Figure 4: Spectral energy distributions. The optical spectra and the NIR photometry come from Cohen & Kuhi (1979) and 2MASS, respectively. Dashed line shows the absorbed black body spectrum having the effective temperature found by (Cohen & Kuhi 1979), coinciding with the J-band photometry, and consistent with the optical spectra. Upper and lower dotted lines show the same black body spectrum for minus and plus 0.5 mag extinction, respectively. We give the resulting bolometric luminosities, including typical ranges for a typical extinction error of 0.5 mag.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{0070fig5.ps} \end{figure}$ Figure 5: H.-R. diagram of LkH $\alpha$ 312 (open dot). Dotted lines show Baraffe et al. (1998) PMS tracks (with the mixing length required to fit the Sun, $l_{\rm mix}=1.9~H_{\rm p}$ , with $H_{\rm p}$ the pressure scale height). Dashed lines show the corresponding isochrones. Black dots mark the position of known classical T Tauri stars in the vicinity of LkH $\alpha$ 312 (Herbig & Bell 1988). The dashed circle marks the position of LkH $\alpha$ 313 in case of binarity. Error bar shows the typical error on the effective temperature (Cohen & Kuhi 1979), and on the bolometric luminosity (see Fig. 4).
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In the text

$\begin{figure} \par\includegraphics[angle=90,width=15.2cm,clip]{0070fig6.ps} \end{figure}$ Figure 6: Chandra background subtracted X-ray light curves of LkH $\alpha$ 312. The time binning is 20 min. Crosses show the X-ray light curve and one-sigma error bars in the energy range 0.5-8 keV. The stripes show the X-ray light curves including one-sigma errors corresponding to the energy ranges 0.5-1.6 keV (soft band), 1.6-2.6 keV (h₁ band), and 2.6-8 keV (h₂ band). Vertical dashed lines and horizontal arrows define the time intervals, $\Delta t_{i}$ , used for the time-dependent spectroscopy. The quiescent, fast rise, and gradual phases correspond to the time intervals $\Delta t_{\rm0}$ , $\Delta t_{\rm 1}$ - $\Delta t_{\rm 2}$ , and $\Delta t_{\rm 3}$ - $\Delta t_{\rm 9}$ , respectively.
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In the text

$\begin{figure} \par\includegraphics[width=8.6cm,clip]{0070fg7a.ps}\hspace*{2mm} \includegraphics[width=8.4cm,clip]{0070fg7b.ps} \end{figure}$ Figure 7: Left panel: hardness ratios of LkH $\alpha$ 312 versus time. The top and bottom panels show respectively the hardness ratio HR1 and HR2, computed from the counts collected in three energy bands (soft, h₁, and h₂, see Fig. 6), with 1.64 $\sigma$ error bars. The grey stripe indicates the average hardness ratio of the quiescent phase. Right panel: hardness ratios of LkH $\alpha$ 312 compared to spectral models. Iso-energy and iso-absorption lines correspond to absorbed optically thin one-temperature plasma spectra model with 0.2 solar abundances, and show the correspondence between the hardness ratio pair (HR1, HR2) and the absorption/temperature pair ( $N_{\rm H}$ , T). The grey area shows the average of two plasma temperatures weighted over the emission measure found from spectral fitting of the quiescent phase (see below Sect. 5.2, Table 4).
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{0070fig8.ps} \end{figure}$ Figure 8: Chandra ACIS-I spectra of LkH $\alpha$ 312. The upper panel displays separately the ACIS-I data for the quiescent (circle mark) and flare phases. The lines are the corresponding models, a two-temperature plasma combined with photoelectric absorption (see Table 4 for details). The lower panel shows the residual $\chi$ of the modeling.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{0070fig9.ps} \end{figure}$ Figure 9: Time-dependent flare spectra. Time interval spectra are plotted from bottom to top with their one-temperature fit (see Table 4 for details) multiplied by an arbitrary constant.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{0070fg10.ps} \end{figure}$ Figure 10: Flare parameters versus time (crosses) compared with the quiescent preflare values (grey stripes) from values in Table 4. The measured values for the quiescent phase are indicated with a dashed-dot line, with a range shown as a grey stripe. Time sampling is defined in Fig. 6. Since the flare spectra can each be fitted with a single temperature, we take for comparison with the quiescent preflare temperature the average of the two plasma temperatures weighted over the emission measures, and for the quiescent preflare emission measure the sum of the two emission measures.
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In the text

$\begin{figure} \par\includegraphics[angle=90,width=8.6cm,clip]{0070fig2.ps} \end{figure}$	Figure 2: ROSAT/HRI X-ray light curve of LkH $\alpha$ 312 obtained from September 5 to October 5, 1997. Crosses show the X-ray light curve and one-sigma error bars in the energy range 0.1-2.4 keV for each observational segment ( $\sim$ 20 min). Background count rates are shown as upper limits when the background subtracted count rate is negative.
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{0070fig8.ps} \end{figure}$	Figure 8: Chandra ACIS-I spectra of LkH $\alpha$ 312. The upper panel displays separately the ACIS-I data for the quiescent (circle mark) and flare phases. The lines are the corresponding models, a two-temperature plasma combined with photoelectric absorption (see Table 4 for details). The lower panel shows the residual $\chi$ of the modeling.
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{0070fig9.ps} \end{figure}$	Figure 9: Time-dependent flare spectra. Time interval spectra are plotted from bottom to top with their one-temperature fit (see Table 4 for details) multiplied by an arbitrary constant.
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