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Rapid variability of the EXor star NY Ori

ATel #8828; D. Lorenzetti (INAF - Osservatorio Astronomico di Roma, Monte Porzio, Italy), A. A.Arkharov, N. Efimova (St. Petersburg Pulkovo Observatory, Russia), T. Giannini, S. Antoniucci, A. Di Paola (INAF - Osservatorio Astronomico di Roma, Monte Porzio, Italy), V. M.Larionov (Astronomical Institute of St. Petersburg University, Russia)
on 16 Mar 2016; 14:22 UT
Credential Certification: Dario Lorenzetti (dloren@mporzio.astro.it)

Subjects: Infra-Red, Variables, Young Stellar Object, Pre-Main-Sequence Star

We report on a rapid brightness variability of the classical EXor star NY Ori observed with the AZT24 1m IR telescope (Campo Imperatore, Italy), as a part of our program EXORCISM (EXOR OptiCal and Infrared Systematic Monitoring - Antoniucci et al. 2013 PPVI; Lorenzetti et al. 2009 ApJ 693, 1056). Our observations covering a period of more than 4 years (since 2011 Oct. to 2015 Dec.) are summarized in the Table below:

JD-2400000.5J(mag)H(mag)K(mag)
55863.610.699.438.50
55868.610.849.468.50
55874.610.469.238.33
55879.610.279.118.17
55881.510.269.298.10
55885.510.599.338.39
56561.510.429.288.41
56577.510.599.128.21
56591.510.068.978.09
56644.410.719.378.41
56728.3 9.388.808.13
56730.310.819.478.48
56736.310.669.458.50
56948.610.759.418.34
56958.610.469.208.28
56959.610.399.138.23
56960.610.489.248.28
56962.610.179.048.16
56963.610.459.248.32
56975.511.039.588.51
57015.510.058.998.14
57339.510.018.968.20
57341.5 9.868.878.14
57343.5 9.648.647.91
57346.6 9.959.238.16
57357.5 9.888.748.14
57358.5 9.968.358.22
57360.510.089.008.21

Magnitude errors are less than 0.03 mag. The light-curves are shown in this figure. To study variations on a short timescale (days) we list below the differences in brightness (expressed as ΔJmag) observed for any pair of subsequent dates spaced by Δt from 1 to 6 days. The fluctuations are significantly larger than the photometric errors. Remarkable is the fading of about 1.4 mag that occurred within just two days (between 56728.3 and 56730.3). Similar fluctuations occur at a lesser extent in H, K bands.

Δt(days)ΔJ (mag)
1 +0.07, -0.09, -0.28, -0.08
2 +0.01, -1.43, -0.31, +0.015, +0.22, -0.12
3 -0.31
4 -0.33
5 -0.15, +0.19
6 +0.38, +0.17, +0.15

Given the random cadence of our observations, we conclude that NY Ori undergoes continuous variations too rapid to be due to disk accretion events. We do not rule out EXor-type variability on longer timescales, but signal that other phenomena (e.g. hotspots onto the stellar surface) may have a major impact. Observations of different EXors on a short timescale are encouraged to ascertain whether or not the NY Ori phenomenology is a typical feature of the entire class.