Current Projects

Periods and classifications of RR Lyrae stars in the globular cluster M15  |  Observational Astro & Computing Tool Development

We present measurements of the periods, amplitudes, and types of 78 RR Lyrae stars in the globular cluster M15 derived from Nickel 1 m telescope observations conducted at Lick Observatory in 2019 and 2020. Of these, two were previously reported but without a determination of the period. In addition, we identify five Cepheid variable stars for which we report three novel period determinations, and a further 35 stars with uncertain classifications and periods. We discuss the development and subsequent application to our data of a new Python package, Period-determination and Identification Pipeline Suite (PIPS), based on a new adaptive free-form fitting technique to detect the periods of variable stars with a clear treatment of uncertainties.

(Hoffman, Murakami, Zheng,, et al. submitted. arxiv: 2008.09778).

Screenshot_2020-08-25 2008 09778 pdf.png

Local Dynamics of Cosmological Scalar Field  |  Modified Gravity

The scalar field for dark energy may exhibit a complicated time- and position-dependent behavior when coupled to matter. A more detailed analysis of intermediate regions connecting local and global phenomena is essential when we test such theories. The preliminary result is presented at the 2020 AAS April Meeting (General oral session T15.8).

Modulation Over 40+ Years: Independent and Combined Observations of RR Lyrae Stars in M15  |  Observational Astrophysics

Our Independent observation and the new determination of period confirms the existing trend of long-term period modulation of RR Lyrae stars in globular cluster M15 (NGC 7078). The preliminary result is presented at 236th APS meeting (iPoster session 132.04).


The Mechanics of Light-Curve Reconstruction  |  Astronomy Data Science

Modern sky-survey type observations are optimized for covering the large area of the sky. Although this provides a large amount of data for the study of variable stars, the cadence of observation is not high enough for short-period variables, such as RR Lyrae. For such data, traditional methods to determine periods, such as Lomb-Scargle Periodogram, do not provide the true value or a good estimation of the uncertainty. We have developed a new algorithm that addresses this issue. The initial study of this algorithm is included in M15 project and presented at 236th APS meeting (iPoster session 132.04: see the poster above).

© 2020 by Yukei Murakami.

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