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Formation of Dust in Nova Sgr 2015b

ATel #7643; Frederick M. Walter (Stony Brook University)
on 16 Jun 2015; 15:58 UT
Credential Certification: Fred Walter (frederick.walter@stonybrook.edu)

Subjects: Infra-Red, Optical, Request for Observations, Nova

Referred to by ATel #: 7748, 7953, 7986, 8133, 8275, 10557

The bright nova N Sgr 2015b (PNV J18365700-2855420) is fading rapidly at optical wavelengths as reported in, for example, VSNet-alert 18731 and AAVSO Alert notice 519. Observations with the SMARTS/Andicam dual-channel photometer on the SMARTS/CTIO 1.3m telescope show a simultaneous brightening at long wavelength. From MJD 57173.8 through MJD 57187.9 the V magnitude faded by 2.0 mag; J faded by 0.92 mag; the Ks magnitude brightened by 0.25 mag. This is likely due to the formation of dust on the line of sight. The dust appears to be optically thick at least through the J-band.

The nova remains bright with V=7.9 and K=4.5 on MJD 57187.9 (2015 June 14 UT). The near-IR magnitudes are suspect because core of the image is currently saturated at H, and in the non-linear part of the response at K, and are best treated as lower limits.

We urge observers with near-IR spectroscopic capability and with near-IR and mid-IR cameras to monitor the formation of the dust.

Meanwhile, we continue to monitor this nova photometrically in BVRIJHK and spectroscopically with the Chiron echelle at R=78,000. There are prominent absorption systems in H-beta, Fe II 517 nm, and Na D at velocities of -700, -800, -1600, and -1800 km/s. Details of the line profiles change nightly, but no gross changes in line profiles seem to coincide with the dust formation.