Stellar PHOTOMETRY ON TIMESCALES OF TENTHS OF SECONDS TO YEARSJeff WilkersonLuther College

MSU MankatoOctober 11, 2007

Our Observations

Science Goals

Data Acquisition and Analysis

Photometric Limits

Variable Star Observations

We image 3 clusters per year: M23 and two othersImage durations: 2 to 12 seconds, unfilteredCampaign durations: 5 to 7 monthsReturn to a cluster at least onceBVRI photometry at least once for color correction to magnitude conversion and knowledge of variable star colorsResult: tens of thousands of images per cluster per yearHow did we get here? What are our goals?Equipment: 12 Meade Schmidt-Cassegrain; Apogee AP6E cameraWhat We Do

All images acquired with a 12 Meade LX200 and Apogee AP6E camera

OUR DATA SETS

ClusterDur. (s)# NightsTotal ImagesDate RangeNGC 6531 (M21)3.52130,00026 June 2002 8 Sept 2002NGC 6514 (M23)3.52545,00019 June 2003 8 Sep. 2003NGC 12910.5915,00011 Aug. 2003 8 Sep. 2003NGC 2682 (M67)2.01435,00025 Feb. 2004 26 April 2004NGC 6694 (M26)9.02028,00024 June 2004 9 Sep. 2004NGC 6514 (M23)2.52045,00023 June 2005 30 Aug. 2005NGC 22867.52228,00024 Jan. 2006 10 April 2006NGC 6514 (M23)5.03749,00028 Mar. 2006 25 Sep. 2006NGC 738010.04044,00012 Jul. 2006 9 Jan. 2007NGC 22867.52944,00031 Oct. 2006 5 Apr. 2007NGC 6514 (M23)2.84991,0009 Mar. 2007 27 Sep. 2007NGC 738010.0~26~30,0005 Jul. 2007 presentNGC 22865.0~2~31003 Oct. 2007 present

Student Participation:Ujjwal JoshiNathan RengstorfAndrea Schiefelbein

Todd BrownBrajesh LacoulKari FrankAlex NugentRobyn SiedschlagSiri ThompsonMatt FitzgeraldHeather LehmannAmalia AndersonHilary Teslow

Zebadiah HowesBuena Vista Univ.

Travis DeJongDordt College

Forrest BishopDecorah High School

Support: Roy J. Carver Charitable Trust (Grant #00-50) Luther College R.J. McElroy Trust/Iowa College Foundation

Our Observational Goals:I. Brief changes in apparent stellar flux Occultation and microlensing events Flare stars

II. Very long timescale changes in stellar luminosity Luminosity stability Solar-like cycles6 Low-amplitude, ultra-long period pulsation

III. Traditional Stellar Variability Surveys of new variable stars Locate detached and semi-detached eclipsing binaries in clusters1 Locate contact eclipsing binaries in clusters2 Period/amplitude variations in contact systems3 Period-to-period variability in long period variables Search for cataclysmic variables in clusters4 Search for transiting planets5 Rotating variable star periods in young clusters7

Wyithe, J.S.B, and Wilson, R.E. 2002, ApJ, 571, 293Rucinski, S.M. 1998, AJ, 116, 2998Paczynski, B., et al. 2006, MNRAS, 368, 1311Mochejska, B.J., et al. 2004, AJ, 128, 312Mochejska, B.J., and Stanek, K.Z. 2006, AJ, 131,1090Lockwood G.W., et al. 1997 ApJ, 485, 780-811Herbst W. and Mundt R., 2005, ApJ, 633, 967-985

010203040506028003000320034003600380040004200Signal (ADU)Negative 5.4 sigma fluctuationDense Object

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M21 Star 5; June 26, 2002

Counts

Signal (ADU)

Estimate the number of occultation events per image:Depends on # of objects in the field and the likelihood of any object hitting a startmin depends on image duration and photometric precision. If a star is occulted for 10% of the image durationAnd s/m=2.5% then a 4s event results.Assume a power law distribution of occulting objects:Event rate at any threshold goes as vx+2 and (tmin)x+1e.g., if x = -4 (as for known KBOs) event rate goes as v-2 and (tmin)-3; a similar analysis for microlensing events yields an event rate proportional to v-2/3.

HR DIAGRAM WITH INSTABILITY STRIPS From Understanding Variable Stars by Percy (Cambridge, 2007)

DATA PROCESSINGAll Analysis done with code developed in IDLCALIBRATIONDark Noise CorrectionFlat FieldingALIGNMENT Use a single frame for entire data setSTAR ID & EXTRACTIONAperture photometry for signal determination256 Background regionsINTRA-NIGHT NORMALIZATIONINTER-NIGHT NORMALIZATIONMAGNITUDE CONVERSION

Aperture Photometry

Frame Normalization

Identify four reference images from throughout the nightCalculate average flux for each star in all four frames this is the reference signalDetermine the signal of each star in the frame to be normalized this is the sample signalCalculate (ref. signal/sample signal) for each starNormalization factor = median of all ratios in (4)

INTRA-NIGHT NORMALIZATION FACTORS Stars appear dimmer lower in the sky due to increased atmospheric scattering.

Stellar color is important in atmospheric scatteringUse Web Version of the BDA catalog for colors (Mermilliod and Paunzen , http://www.univie.ac.at/webda//).

How is our photometry? Define Short-term Photometric Resolution (STPR) as s/m for a Gaussian fit to a histogram of several hundred signal measurements for a given star. At large signal values STPR approaches a constant (plateau) value determined by our frame normalization, itself limited by scintillation. For faint stars STPR increases as signal-1. In between STPR increases as signal-1/2. Counting statistics of the stellar signal measurement dominate STPR in this region.Functional fits shown of form: STPR=[(C1) + (C2signal-1/2) + (C3signal)2]1/2

Photometric resolution is determined largely by atmospheric scintillation and improves with the altitude of the observation.

We define long-term photometric resolution (LTPR) as s/m for the nightly average signal measure of a given star over an entire campaign

On each night we look at a set of several hundred brighter stars with suspected variables remove. We measure the scatter of the nightly mean divided by the campaign mean. In this way we identify nights of unacceptable photometric quality.

Use Web Version of the BDA catalog for magnitudes (Mermilliod and Paunzen , http://www.univie.ac.at/webda//). Find an empirical color equation for the system and apply it.

Observation of Variable Stars in the M23 FieldTo date we have identified 36 variables in the M23 field. Of these, one appears in the GCVS (http://www.sai.msu.su/groups/cluster/gcvs/gcvs/) as a semi-regular variable and one more appears as an eclipsing binary in the ASAS catalog (http://archive.princeton.edu/~asas/). The remainder appear to be newly discovered variables.

Of the variable stars observed we tentatively classify 4 of them as long period (P>1 day) eclipsing binaries, 3 as short period eclipsing binaries and the remainder as long period pulsating stars.

COLOR/MAGNITUDE DIAGRAMSThe left diagram is constructed for the brightest stars in the field, including main sequence cluster members. The middle diagram includes the middle 480 stars, ranked in intensity. The right diagram includes only the faintest stars in the field. As expected, the long period pulsating stars are among the reddest in the field. The eclipsing binaries lie near the M23 main sequence. BVRI color measurements were made utilizing a nearby low-precision standard field from the Loneos Catalogue. (Henden, A., ftp://ftp.nofs.navy.mil/pub/outgoing/aah/photom/ubvri.cat)

Long Period Eclipsing Binaries

Short Period Eclipsing BinariesPeriod = 0.32866 daysPeriod = 0.54550 daysPeriod = 0.20730 days

Search for Period Changes in Short Period Eclipsing Binaries

Sample Light Curves of Long Period VariablesStar 338Star 414Star 317Star 850

Sample Light Curves of Long Period VariablesStar 294Star 1223Star 981Star 1654

Long Period Variables M23 Field

Other Interesting StarsShort-period pulsating variableLong-term drift star candidate

CONCLUSION We survey three open cluster regions per year, collecting 50,000 or more images of each field over a ~six month time span.

Can detect variability on timescales of minutes to years with a precision of a few percent or better

By summing images can survey stars from 8th to 19th magnitude.

Have found many new variable stars in field of open cluster M23; these provide a rich set of objects for long-term study

Have completed a second campaign on NGC2286; so many more clusters to look at.