Transcript Search For Trans-Neptunian Objects Using COROT

Search For Trans-Neptunian Objects Using COROT Asteroseismology Lightcurves
Chih-Yuan Liu(1,2,3), Alain Doressoundiram(1), Françoise Roques(1), Hsiang-Kuang Chang(2,3), Michel Auvergne(1)
(1) Observatoire de Paris, LESIA, 92195 Meudon cedex, France
(2) Department of Physics, National Tsing Hua University, Hsinchu, 30013, Taiwan
(3) Institute of Astronomy, National Tsing Hua University, Hsinchu, 30013, Taiwan
Abstract
Data Processing
Trans-Neptunian Objects (TNOs) are the witnesses of the early Solar System
and can provide astronomers much valuable knowledge about their formation
and dynamical evolution. The current understanding of their properties, however,
such as the total mass, orbital parameters and the size distribution, is still far
from complete.
Here we will present a study of re-examination of COROT asteroseismology
lightcurves for the search of TNOs. COROT (COnvection ROtation and planetary
Transits) was launched on December 2006. Its two major goals are to search for
extrasolar planets and to perform asteroseismology by measuring solar-like
oscillations in stars. For our purpose, we will only use asteroseismology data
which has the integration time of 1 second. The total observation time avaliable
in this work is about 82032.9 star-hours. We will analyze these fast photometry
lightcurves data to search for serendipitous occultations by passing TNOs.
Occultation method is a powerful tool to detect small Solar System bodies
through their diffraction phenomenon (see Roques et al, 2000, Icarus, 147, 530).
The method of occultation will allow to reveal the existence of TNOs which have
sizes about several kilometers.
Discussion & Future Works
Discussion
TABLE2: COROT ASTEROSEISMOLOGY N1 DATA EMPLOYED
Segment
run code
date begin
date end
STAR HOURS
1
IRa01
01/31/2007 11:06:34
04/02/2007 07:12:15
12455.77
2
SRc01
04/11/2007 15:07:52
05/09/2007 07:13:21
5799.25
3
LRc01
05/11/2007 13:10:19
10/14/2007 23:59:59
33460.73
4
LRa01
10/18/2007 08:57:24
03/03/2008 09:49:35
28665.71
5
SRa01
03/05/2008 22:34:26
03/31/2008 07:43:58
5422.20
6
SRa02
10/08/2008 22:44:36
11/12/2008 08:29:29
7450.72
7
LRa02
11/13/2008 22:49:46
03/11/2009 10:31:38
25137.60
8
LRc03
04/01/2009 20:49:11
07/02/2009 03:53:58
9813.25
9
LRa03
10/01/2009 20:57:34
03/01/2010 08:37:24
16203.11
TOTAL STAR HOURS
•In
our COROT data search, there are 519869933 1-sec bins, and the random
probability for -6.5σ is about 3x10-11. We probably should see a few bins below 5σ. Since there is no detection, it may mean that the fluctuation in our data is not
only random noise but also some other things so that the variation estimated
from the data itself is larger than the random fluctuation.
•A way
Figure 1a
Figure 1b
to derive constraints on the Kuiper Belt from this zero-detection without
hypothesis on the size distribution, is to consider two parameters, the scanned
sky area Sc and the size of detectable objects, rd.The observation gives an upper
limit of the density of objects larger than the detectable object (rd) on the scanned
sky area (Sc deg2). The Figure-4 gives the size distribution of objects in which is
plotted the last estimates of Schlichting from HST data, some upper-limits from
other surveys, and our 11 possible detections thus eliminates any power-law
size distribution steeper, setting a stringent constrain to the number density of
TNOs.
144,408.34
COROT Asteroseismology N1 Data
Introduction
Currently we have 165 COROT asteroseismology N1 lightcurves in this work.
Based on the run codes of the observations, we divide these lightcurves into 9
segments, each segment has 10 to 15 lightcurves. The corresponding run codes
and accumulated observing time for these 9 segments are shown in Table 2.
Objective
Properties of 70 AN1 Lightcurves
The direct observation for TNOs is only possible for larger objects with diameters
above several tens of kilometers. Small TNOs could only be found when they
obscure background stars. Since occultations for these invisible objects are not
predictable, the blind search for occultations in lightcurves of monitored stars
may be possibly useful. We will apply this method to COROT asteroseismology
data for the detection of small TNOs.
COnvection, ROtation & planetary Transits
COROT has a polar inertial circular orbit (90-degree inclination) at an altitude of
896 kilometers. The apogee and perigee are respectively 911 and 888
kilometers. The orbital period is 6184 seconds. Twice a year, when the Sun gets
closer to the orbit plane and is about to blind the telescope, the spacecraft
performs a reversal attitude maneuver, dividing the year into two 6-month
periods of observation (by convention, summer and winter). There are four
observing runs (alternately 20 and 150 days) for a year. During the northern
summer it will observe in an area around Serpens Cauda and during the winter it
will observe in Monoceros. The first four run codes of COROT and their
observing area are listed below:
•IRa01: in the constellation of Monoceros
•SRc01: in the constellation of Serpens Cauda
•LRc01: in the constellation of Aquila
LRa01:
in
the
constellation
of
Monoceros
(LR stands for long run, SR for short run, IR for initial run and the small letters a
and c for galactic anti-centre or galactic-centre direction).
In this work, we will use COROT asteroseismology (level-1) data from 4
observing runs made between 31 January 2007 and 12 November 2008 to
search for serendipitous occultations. In each field of view there will be one main
target star for the asteroseismology as well as up to nine other targets. Detail
information for long observing run and short observing run is shown in Table 1a
& Table 1b.
TABLE 1a
TABLE 1b
•For our data sets, there are 79 stars monitored by COROT.
•Vmag of those 79 stars: 4.77 ~ 9.48.
•S/N ~ 360 for a V=7.36 star.
•The integration time for COROT AN1 lightcurves is 1 sec.The information about
the “cause of the rejection” (OVER) is included in the binary table of the
lightcurve. The value of OVER indicates the status of the measurement. For our
data reduction, we choose the bins with the value of OVER equals zero only.
‘OVER = 0’ means that data within this 1-sec bin-size is not affected by crossing
SAA, energetic particle impacts, glitches or other status changing of the satellite.
•After screening, the observing time of the longest and shortest lightcurves are
131.5 days and 411 seconds respectively.
Search algorithm
The algorithm we used here is very similar with the method applied to the search
of TNO occultation in X-rays: we derive the deviation distributions for all COROT
AN1 lightcurves, set the search criteria and look for the dips. The steps of our
search algorithm are described below:
Table 3
Figure 2
bin size = 1 sec
• Estimate the deviation: For each bin of a lightcurve, we apply a 60-sec running
window on it and calculate the statistic values of the intensity of star. We then
reinterpret the intensity in the unit of standard deviations instead of the electron
numbers in order to plot the deviation distribution. Figure-1a is the screened
lightcurve of the star HD49933 observed in COROT LRa01 run. Figure-1b is
replotted with the intensity in the unit of σ.
• Set criteria: We perform a linear fitting for the negative part of the StandardScore (SS) distribution from number of bins 1000 to 3. The bins with SS smaller
than threshold T(n) will be the outliers (see Figure-3). T(n) value depends on the
window size: between -6.2 and -9.3.
• Examine the reality of dips: Not every dip we found will be due to an
astronomical occultation. For example, if the dip could be found at the same
epoch in other lightcurves, it will be an artificial event due to occurring in the
terrestrial environment (airplane, satellite, seeing effects etc.)
Figure 4
An example of the deviation distributions of the COROT 519859933 data bins.
Figure 3
Preliminary Results
COROT ASTEROSEISMOLOGY OBSERVATION PROPERTIES
Credit to http:://smsc.cnes.fr/COROT/sismologie.htm
This selection method is applied to Standard-Score distributions with 2n values from 20 to 180.
This research leads to 20 outliers.To validate the reality of these events, we examine the raw data with the assistance
from COROT team members:
➡7 outliers are excluded because they correspond to de-focusing processes or cross-talk interactions between
seismology and exoplanetary CCDs.
➡The others 13 outliers are identified as Possible Occultation Events, the so-called POEs (Figure 4).
➡All the outliers detected with 2n values larger than 180 are artifacts. See Table 3 for the properties of the 20 outliers.
References
1.Zhang et al. The TAOS Project: Results From Seven Years of Survey Data.
eprint arXiv (2013) vol. 1301 pp. 6182Chang et al. Search for serendipitous
trans-Neptunian object occultation in X-rays. Monthly Notices of the Royal
Astronomical Society (2013) vol. 429 pp. 1626Schlichting et al. Measuring the
Abundance of Sub-kilometer-sized Kuiper Belt Objects Using Stellar
Occultations. The Astrophysical Journal (2012) vol. 761 pp. 150Ofek and
Nakar. Detectability of Oort Cloud Objects Using Kepler. The Astrophysical
Journal Letters (2010) vol. 711 pp. L7Roques et al. Exploration of the Kuiper
Belt by High-Precision Photometric Stellar Occultations: First Results. The
Astronomical Journal (2006) vol. 132 pp. 819