Investigation of LSST Periodic Variable Star Lightcurve Recovery
1
LSST will discover millions of periodic variable stars, which can be valuable as standard candles to allow mapping of Galactic structure. We have investigated LSST's capability to recover RR Lyrae lightcurve shapes and periods as a function of stellar distance and LSST survey length. A LSST simulation tool was used to sample model RR Lyrae lightcurves of varying metallicity, mass, luminosity, temperature, and oscillation mode. The simulation tool returns each lightcurve as it would have been sampled by LSST, including realistic limiting magnitudes and photometric scatter. The stars were placed on the sky to sample the complete set of LSST cadences, including universal cadence fields, overlap regions, deep drilling fields, solar system fields, and Milky Way fields. The lightcurve recovery capability was investigated for LSST survey lengths of 1 to 10 years, and for RR Lyrae stars with g-band brightnesses ranging from 17th to 26th magnitude. We attempt to recover both the input periods and Fourier-based shapes of the phased lightcurves. A major objective of modern astrophysics is to understand the details of galaxy formation and evolution. Periodic variable stars such as RR Lyrae, δ -Scuti, and Cepheids provide a means for mapping the galactic structure and accretion history through relating stellar (e.g. metallicity) and pulsational parameters (period and amplitude of pulsation) with evolutionary parameters (e.g. luminosity). We report initial results from our study to investigate LSST's ability to recover the pulsational characteristics of periodic variables. Introduction Periodic Variable Lightcurves We have obtained model bolometric RR Lyrae lightcurves that have been transformed into the Sloan Digital Sky Survey (SDSS) ugrizy photometric system, which is similar to the system adopted for LSST photometry. The non-linear non-local time-dependent convective RR Lyrae stellar models used for our study were originally presented in Marconi et al. (2006). The models span a range of metallicity, Helium content, stellar mass, and luminosity as shown in the table below. Fundamental and first overtone pulsators were calculated for several effective stellar temperatures. Shown below are the model lightcurves u(blue), g(green), r(red), i(magenta), z(black) for a Z = 0.001, M/M = 0.65, log L/log L = 1.51, T = 6500 K, and g-mag = 22 fundamental pulsator with a period 3.60250266 × 10 4 seconds. Conclusions The preliminary results reported here illustrate our approach for simulating LSST’s ability to recover the lightcurves of periodic variable stars. The main challenge to this work is the intense computational time required for observation simulations and analyses. Future work will include the completion of this study utilizing RR Lyrae variables with apparent magnitudes ranging from mag 17 to mag 26. We plan to extend the study to other types of periodic variable stars including Cepheid variables and δ-Scuti stars. Ultimately, we plan to simulate LSST observations of several classes of objects in the astronomical variability tree (Eyer & Mowlavi 2007). Acknowledgements This material is based upon work supported by a grant from the National Science Foundation. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. Thanks to the LSST Corporation, University of Washington, and the Department of Energy Office of Science who collaborated with the NSF to arrange, fund, and coordinate this research project. We have begun simulating LSST observations for five regions of the sky that LSST will observe with different cadences. A subset of 1007 fields were selected in the regions that have been designated Universal Cadence fields (N = 400), Overlap fields (N = 400), North Ecliptic fields (N = 100), Milky Way fields, (N = 100) and Deep Drilling fields (N=7). Each ugrizy RR Lyrae model (N = 216) was placed in each field and sampled with the LSST simulation tool, which returned realistic limiting magnitudes and photometric scatter. The stellar magnitudes ranged from 17th to 26th with Δ = 0.5 mag, for survey lengths from 1 to 10 years with Δt = 1 year. Multi-cadence LSST Observation Simulations Analysis The frequency of the unequally spaced time-sampled and noised periodic data was fit using periodograms and least squares estimation methods (Reiman 1994). The phased simulated data was then fit via a Chi-square minimization by a Fourier series of the form: The calculated period and Fourier parameters were compared with the input values. We show phased and fitted 10-year lightcurves below. mt () = A 0 + A i cos 2πit − t 0 ( ) + φ i [ ] i ∑ References Eyer, L. & Mowlavi, N. 2007, “Variable Stars across the HR Diagram,”JPhCS 118, 2010. Marconi et al. 2006, “Predicted properties of RR Lyrae stars in the Sloan Digital Sky Survey photometric system,” MNRAS 371, 1503. Reimann, J. D. 1994, “Frequency Estimation Using Unequally Spaced Astronomical Data,” Ph.D. Thesis, University of California, Berkeley 23.2 23.6 24.0 24.4 24.6 24.2 23.8 23.4 0 0. 2 0. 4 0. 6 0. 8 1.0 21.0 21.5 22.0 22.5 23.0 23.5 0 0.2 1.0 0.8 0.6 0.4 Apparent magnitude phase Milky Way field at g-mag = 22 for 10-years date date Apparent magnitude r z g i g i y r z UC-field simulations at g-mag = 24 Apparent magnitude date date g i y u r z Apparent magnitude date date UC-field simulations at g-mag = 22 North Ecliptic field at g-mag = 22 for 10-years r z g i date date Deep Drilling field at g-mag = 22 for 10 years u r z g i y date date Apparent magnitude Overlap field at g-mag = 22 for 10 years u r z g i y date date Apparent magnitude Apparent magnitude North Ecliptic g-band mag = 24 23.2 23.6 24.0 24.4 24.8 23.4 23.8 24.2 24.6 0 0. 2 0. 4 0. 6 0. 8 1.0 Deep Drilled g-band mag = 24 Overlap g-band mag = 24 23.2 23.6 24.0 24.4 24.6 24.2 23.8 23.4 0 0. 2 0. 4 0. 6 0. 8 1.0 Universal Cadence g-band mag = 24 23.2 23.6 24.0 24.4 24.6 24.2 23.8 23.4 0 0.2 0.4 0. 6 0. 8 1.0 The figures above illustrate LSST’s lightcurve recovery capability for g-band observations at mag = 24. The RR Lyra model utilized provided a substantial challenge as it has a fast rise time and an inversion in the light curve near peak magnitude. In all but the North Ecliptic field, recovered periods matched input periods to better than 1%. Overlap r-band Survey length (years) 1 2 3 4 5 6 7 8 9 10 Success rate (%) apparent magnitude 100 80 60 40 20 Overlap g-band 1 2 3 4 5 6 7 8 9 10 Survey length (years) 100 80 60 40 20 0 Success rate (%) apparent magnitude Deep Drilling r-band 1 2 3 4 5 6 7 8 9 10 Survey length (years) 100 80 60 40 20 0 Success rate (%) apparent magnitude Deep Drilling g-band 1 2 3 4 5 6 7 8 9 10 Survey length (years) 100 80 60 40 Success rate (%) apparent magnitude Results To date, we have completed a subset of our intended number of LSST observation simulations of RR Lyrae variable stars. These simulations were for stars with apparent magnitudes ranging from mag 17.0 to 25.0. The ability to successfully recover the pulsational periods and lightcurve shapes was field and filter dependent. For example, r-band Universal cadence fields, Overlap fields, and Deep Drilling fields LSST successfully recovered (within 1% of input value) the stellar pulsation periods and lightcurve shapes for all survey lengths with magnitudes ranging up to mag 24.0. For stars dimmer than mag 24.0, shorter survey lengths degraded the ability to correctly recover the pulsational periods and lightcurve shapes. For the Milky Way and North Ecliptic fields the successful recovery of periods and lightcurves began to degrade beyond magnitude 23. The NE fields do not observe often in the u- and y-bands and do not return those data for most magnitudes. The plots below show the rate of success for period recovery in g- and r-band.
Transcript of Investigation of LSST Periodic Variable Star Lightcurve Recovery
LSST will discover millions of periodic variable stars, which can be valuable as standard candles to allow mapping of Galactic structure. We have investigated LSST's capability to recover RR Lyrae lightcurve shapes and periods as a function of stellar distance and LSST survey length. A LSST simulation tool was used to sample model RR Lyrae lightcurves of varying metallicity, mass, luminosity, temperature, and oscillation mode. The simulation tool returns each lightcurve as it would have been sampled by LSST, including realistic limiting magnitudes and photometric scatter. The stars were placed on the sky to sample the complete set of LSST cadences, including universal cadence fields, overlap regions, deep drilling fields, solar system fields, and Milky Way fields. The lightcurve recovery capability was investigated for LSST survey lengths of 1 to 10 years, and for RR Lyrae stars with g-band brightnesses ranging from 17th to 26th magnitude. We attempt to recover both the input periods and Fourier-based shapes of the phased lightcurves.
A major objective of modern astrophysics is to understand the details of galaxy formation and evolution. Periodic variable stars such as RR Lyrae, δ -Scuti, and Cepheids provide a means for mapping the galactic structure and accretion history through relating stellar (e.g. metallicity) and pulsational parameters (period and amplitude of pulsation) with evolutionary parameters (e.g. luminosity). We report initial results from our study to investigate LSST's ability to recover the pulsational characteristics of periodic variables.
Introduction
Periodic Variable Lightcurves We have obtained model bolometric RR Lyrae lightcurves that have been transformed into the Sloan Digital Sky Survey (SDSS) ugrizy photometric system, which is similar to the system adopted for LSST photometry. The non-linear non-local time-dependent convective RR Lyrae stellar models used for our study were originally presented in Marconi et al. (2006). The models span a range of metallicity, Helium content, stellar mass, and luminosity as shown in the table below. Fundamental and first overtone pulsators were calculated for several effective stellar temperatures. Shown below are the model lightcurves u(blue), g(green), r(red), i(magenta), z(black) for a Z = 0.001, M/M = 0.65, log L/log L = 1.51, T = 6500 K, and g-mag = 22 fundamental pulsator with a period 3.60250266 × 104 seconds.
Conclusions The preliminary results reported here illustrate our approach for simulating LSST’s ability to recover the lightcurves of periodic variable stars. The main challenge to this work is the intense computational time required for observation simulations and analyses. Future work will include the completion of this study utilizing RR Lyrae variables with apparent magnitudes ranging from mag 17 to mag 26. We plan to extend the study to other types of periodic variable stars including Cepheid variables and δ-Scuti stars. Ultimately, we plan to simulate LSST observations of several classes of objects in the astronomical variability tree (Eyer & Mowlavi 2007).
Acknowledgements This material is based upon work supported by a grant from the National Science Foundation. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.
Thanks to the LSST Corporation, University of Washington, and the Department of Energy Office of Science who collaborated with the NSF to arrange, fund, and coordinate this research project.
We have begun simulating LSST observations for five regions of the sky that LSST will observe with different cadences. A subset of 1007 fields were selected in the regions that have been designated Universal Cadence fields (N = 400), Overlap fields (N = 400), North Ecliptic fields (N = 100), Milky Way fields, (N = 100) and Deep Drilling fields (N=7). Each ugrizy RR Lyrae model (N = 216) was placed in each field and sampled with the LSST simulation tool, which returned realistic limiting magnitudes and photometric scatter. The stellar magnitudes ranged from 17th to 26th with Δ = 0.5 mag, for survey lengths from 1 to 10 years with Δt = 1 year.
Multi-cadence LSST Observation Simulations Analysis The frequency of the unequally spaced time-sampled and noised periodic data was fit using periodograms and least squares estimation methods (Reiman 1994). The phased simulated data was then fit via a Chi-square minimization by a Fourier series of the form:
The calculated period and Fourier parameters were compared with the input values. We show phased and fitted 10-year lightcurves below.
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m t( ) = A0 + Ai cos 2πi t − t0( ) + φi[ ]i∑
References Eyer, L. & Mowlavi, N. 2007, “Variable Stars across the HR Diagram,”JPhCS 118,
2010.
Marconi et al. 2006, “Predicted properties of RR Lyrae stars in the Sloan Digital Sky Survey photometric system,” MNRAS 371, 1503.
Reimann, J. D. 1994, “Frequency Estimation Using Unequally Spaced Astronomical Data,” Ph.D. Thesis, University of California, Berkeley
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Results To date, we have completed a subset of our intended number of LSST observation simulations of RR Lyrae variable stars. These simulations were for stars with apparent magnitudes ranging from mag 17.0 to 25.0. The ability to successfully recover the pulsational periods and lightcurve shapes was field and filter dependent. For example, r-band Universal cadence fields, Overlap fields, and Deep Drilling fields LSST successfully recovered (within 1% of input value) the stellar pulsation periods and lightcurve shapes for all survey lengths with magnitudes ranging up to mag 24.0. For stars dimmer than mag 24.0, shorter survey lengths degraded the ability to correctly recover the pulsational periods and lightcurve shapes. For the Milky Way and North Ecliptic fields the successful recovery of periods and lightcurves began to degrade beyond magnitude 23. The NE fields do not observe often in the u- and y-bands and do not return those data for most magnitudes. The plots below show the rate of success for period recovery in g- and r-band.
