On the influence of component orbital motion on the photometric variability of DF Tau

TL;DR

DF Tau, a young binary CTTS, exhibits century-scale photometric variability that the authors test against accretion-rate changes modulated by orbital motion. They refine the orbital solution with two independent methods, obtaining $P \approx 52.9$ yr, $T_0 \approx 2024.0$–$2024.6$, $e \approx 0.172$, and $i \approx 40.4^\circ$, and compare the secular light curve to the evolving component separation. The analysis reveals three brightness maxima that do not track the predicted cycle of separation, a pattern also reported in other young binaries, challenging current theories of disk–binary interaction in systems without a circumbinary disk. The work underscores the need for higher-resolution imaging and revised theoretical models to understand accretion variability in DF Tau and similar systems.

Abstract

Based on the analysis of the long-term light curve of the young binary DF Tau spanning approximately 125 years, we infer that its brightness variations are associated with changes in the accretion rate from the circumstellar protoplanetary disk onto the primary. We have also substantially improved the orbital parameters of DF Tau, which enables us to align its secular light curve with the evolution of the binary's component separation. The relationship between the long-term brightness variations and the orbital motion of DF Tau, if present, appears to be inconsistent with theoretical predictions. Notably, similar discrepancies between theory and observations are also seen in other young binary systems. Furthermore, the source of the polarized radiation in the optical range is found to be located at a distance of $\lesssim 0.5^{\prime\prime}$ from the star, with the polarization variability showing no dependence on the orbital phase.

Figures (5)

• Figure 1: The lower-left panels show the marginal posterior probability distributions of the orbital parameters obtained with Method M2. The one-dimensional histograms along the diagonal (from top left to bottom right) represent the marginal distribution of each parameter. The off-diagonal panels, colored by intensity, display the two-dimensional marginal posterior density for each pair of parameters, with darker shades indicating higher probability density. The inset in the upper-right corner shows the orbit of the companion DF Tau B relative to the primary star DF Tau A. The solid and dashed lines indicate the portions of the orbit lying above and below the plane of the celestial sphere, respectively; thus, $N_1$ and $N_2$ denote the ascending and descending nodes of the orbit. The vector ${\bf n}$ indicates the orientation of the orbital angular momentum.
• Figure 2: Historical light curve of DF Tau in the $B$ and $V$ bands (upper and middle panels, respectively). Data points are colored and symbol-coded according to the source: (1) Lamzin-2001, (2) Grankin-07, (3) our observations at the Crimean Astrophysical Observatory (CrAO), (4) AAVSO, (5) Allen-2017, (6) our observations at CMO, (7) outbursts in 1918 Tsesevich-1973 and 2000 Li-2001, and (8) Kutra-2025. The lower panel shows the time evolution of the component separation, normalized to the system's semi-major axis. Red crosses mark the mean $B$-band brightness within each time interval, excluding the 1918 and 2000 outbursts. Blue arrows indicate the epochs of the 1918, 2000, and 2023 outbursts. See text for details.
• Figure 3: The dependence of the standard deviation $\sigma_B$ on the mean brightness $\bar{B}$ for DF Tau, derived from the time intervals indicated in the lower panel of Figure \ref{['fig:lcurve']}, excluding the 1918 and 2000 outbursts. The red line shows a least-squares fit to the data; the correlation coefficient $r$ between $\sigma_B$ and $\bar{B}$ is also indicated. See text for details.
• Figure 4: Top: Image of DF Tau in polarized intensity, reconstructed using differential speckle polarimetry. Bottom: Point spread function used in the reconstruction (shown for illustration of angular resolution).
• Figure 5: Histogram of the polarization angle distribution of DF Tau, constructed from the data of Shakhovskoj-2006. The dashed line shows the expected polarization orientation from the jet. The solid line shows the expected polarization orientation from the disk.