New active asteroid (6478) Gault

ABSTRACT Main-belt asteroid (6478) Gault was observed to show cometary features in early 2019. To investigate the cause, we conducted BVR observations at Xingming Observatory, China, from 2019 January to April. The two tails were formed around 2018 October 26–November 8, and 2018 December 29–2019 January 8, respectively, and consisted of dust grains of ≳20 $\mu$m to 3 mm in radius ejected at a speed of 0.15 ± 0.05 m s−1 and following a broken power-law size distribution bending at grain radius ∼70 $\mu$m (bulk density 1 g cm−3 assumed). The total mass of dust within a 104 km-radius aperture around Gault declined from ∼9 × 106 kg since 2019 January at a rate of 2.28 ± 0.07 kg s−1, but temporarily surged around 2019 March 25, because Earth then crossed the orbital plane of Gault, near which the ejected dust was mainly distributed. No statistically significant colour or short-term light-curve variation was seen. Nonetheless we argue that Gault is currently subjected to rotational instability. Using the available astrometry, we did not detect any non-gravitational acceleration in the orbital motion of Gault.

An extensive photometric study of the Blazhko RR Lyrae star RZ Lyr*

The analysis of recent, extended multicolour CCD and archive photoelectric, photographic and visual observations has revealed several important properties of RZ Lyr, an RRab-type variable exhibiting large-amplitude Blazhko modulation. On the time base of ∼110 yr, a strict anticorrelation between the pulsation- and modulation-period changes is established. The light curve of RZ Lyr shows a remarkable bump on the descending branch in the small-amplitude phase of the modulation, similarly to the light curves of bump Cepheids. We speculate that the stellar structure temporally suits a 4:1 resonance between the periods of the fundamental and one of the higher order radial modes in this modulation phase. The light-curve variation of RZ Lyr can be correctly fitted with a two-modulation-component solution; the 121-d period of the main modulation is nearly but not exactly four times longer than the period of the secondary modulation component. Using the inverse photometric method, the variations in the pulsation-averaged values of the physical parameters in different phases of both modulation components are determined.

Photometric and spectroscopic study of candidate Be stars in the Magellanic Clouds

Mennickent et al. (2002) presented a catalogue of 1056 Be star candidates in the Small Magellanic cloud (SMC) by studying light curve variation using OGLE II data base. They classified these Be star candidates of the SMC in four categories: Type 1 stars showing outbursts (139 stars); Type 2 stars showing sudden luminosity jumps (154 stars); Type 3 stars showing periodic or near periodic variations (78 stars); Type 4 stars showing light curves similar to Galactic Be stars (658 stars). They suggested that Type 4 could be Be stars. On the other hand, they suggested that Type-3 stars may not be linked to the Be star phenomenon at all.

A Joint Framework for Consistently Pricing Interest Rates and Interest Rate Derivatives

Dynamic term structure models explain the yield curve variation well but perform poorly in pricing and hedging interest rate options. Most existing option pricing practices take the yield curve as given, thus having little to say about the fair valuation of the underlying interest rates. This paper proposes an m + n model structure that bridges the gap in the literature by successfully pricing both interest rates and interest rate options. The first m factors capture the yield curve variation, whereas the latter n factors capture the interest rate options movements that cannot be effectively identified from the yield curve. We propose a sequential estimation procedure that identifies the m yield curve factors from the LIBOR and swap rates in the first step and the n options factors from interest rate caps in the second step. The three yield curve factors explain over 99% of the variation in the yield curve but account for less than 50% of the implied volatility variation for the caps. Incorporating three additional options factors improves the explained variation in implied volatilities to over 99%.