# Summary of K2 Program GO8041

Title: Extragalactic Cepheids in the Local Group member IC 1613

PI: Molnar, Laszlo (Konkoly Observatory)
CoIs: Pal, Andras; Neilson, Hilding; Plachy, Emese; Szabo, Robert

Research goals

IC 1613 is an isolated irregular dwarf galaxy within the Local Group. Thanks to its relative proximity (730 kpc), low foreground reddening, and the large number of Cepheid stars in it, it has been the subject of several studies during the past century (see, e.g. Sandage 1971, Bernard et al. 2010, and references therein). It played a key role in the discovery of the different stellar populations and the calibration of the Cepheid period-luminosity relation.  

Although Cepheid stars have been extensively studied in the past, space photometric observations uncovered some new, puzzling details about them. Results from Kepler and MOST showed that significant jitter occurs in the light curve shape and period (Derekas et al. 2012, Evans et al. 2015). Some stars exhibit a low-amplitude additional modes of unknown origin. One mode in particular is strongly linked to the first overtone with a period ratio of ~0.6-0.64, across variable star classes (Moskalik 2014). This mode was first discovered in classical Cepheids in the Large Magellanic Cloud, and later in RR Lyrae stars too. We recently identified the same mode in the K2 observations of first-overtone galactic Cepheids. 

We expect that a significant portion of the IC 1613 sample will be measured accurately enough and long enough (for several pulsation cycles) that we will be able to search for the signs of period jitter and additional modes. The results will then be directly comparable to the K2 observations of Cepheids in the Milky Way. 

Targets

IC 1613 will be observable in its entirety during Campaign 8. It contains about 200 known Cepheids between 19 and 23 magnitudes in V, of which 30 are brighter than 21 magnitudes. The experiences with the faint trans-Neptunian objects (Pál et al. 2015) and with the RR Lyrae stars in the dwarf galaxy Leo IV (Molnár et al., in prep.) indicate that meaningful photometry can be extracted even for targets as faint as 21-23 magnitudes. The brightest stars pulsate with periods comparable with the campaign length (20-40 days), but most stars have periods shorter than 10 days and therefore several consecutive pulsation cycles will be observable. 

Beside the classical Cepheids, a few W Vir stars (Type-II Cepheids) are also known in the galaxy. Furthermore, an extended mosaic covering the area may also allow for extracting additional bright sources to explore other science cases.

Observing strategy and research team

Given the faintness of the targets, we propose to observe all targets in long cadence mode. Cepheids populate IC 1613 quite densely, compared to the resolution of Kepler. Their spatial distribution justifies tiling a large part of the galaxy with a continuous mosaic, instead of individual, potentially overlapping target pixel masks. Image subtraction techniques will be applied to extract the photometry of the faint but strongly variable stars. We will employ the same pipelines that we used to process the TNO and Leo IV targets.

Our team includes members of the KASC RR Lyrae and Cepheid Working Group, and consists of researchers who are experts in pixel photometry and crowded fields, light curve analysis, and hydrodynamic modelling of stellar pulsation. We routinely analyse Kepler and K2 data and have published multiple papers on related topics. 

References
Bernard, E.J. et al., 2010, ApJ, 712, 1259
Derekas, A. et al., 2012, MNRAS, 425, 1312
Evans, N. R. et al., 2015, MNRAS, 446, 4008
Moskalik, P., 2014, IAUS, 301, 249
Pál, A., et al., 2015, ApJ, 804, L45
Sandage, A., 1971, ApJ, 166, 13

# Targets requested by this program that have been observed (98)
EPIC ID, RA (J2000) [deg], Dec (J2000) [deg], magnitude, Investigation IDs
200068656, , , , GO8041_LC|LC_IC1613_TILE
200068657, , , , GO8041_LC|LC_IC1613_TILE
200068658, , , , GO8041_LC|LC_IC1613_TILE
200068659, , , , GO8041_LC|LC_IC1613_TILE
200068660, , , , GO8041_LC|LC_IC1613_TILE
200068661, , , , GO8041_LC|LC_IC1613_TILE
200068662, , , , GO8041_LC|LC_IC1613_TILE
200068663, , , , GO8041_LC|LC_IC1613_TILE
200068664, , , , GO8041_LC|LC_IC1613_TILE
200068665, , , , GO8041_LC|LC_IC1613_TILE
200068666, , , , GO8041_LC|LC_IC1613_TILE
200068667, , , , GO8041_LC|LC_IC1613_TILE
200068668, , , , GO8041_LC|LC_IC1613_TILE
200068669, , , , GO8041_LC|LC_IC1613_TILE
200068670, , , , GO8041_LC|LC_IC1613_TILE
200068671, , , , GO8041_LC|LC_IC1613_TILE
200068672, , , , GO8041_LC|LC_IC1613_TILE
200068673, , , , GO8041_LC|LC_IC1613_TILE
200068674, , , , GO8041_LC|LC_IC1613_TILE
200068675, , , , GO8041_LC|LC_IC1613_TILE
200068676, , , , GO8041_LC|LC_IC1613_TILE
200068677, , , , GO8041_LC|LC_IC1613_TILE
200068678, , , , GO8041_LC|LC_IC1613_TILE
200068679, , , , GO8041_LC|LC_IC1613_TILE
200068680, , , , GO8041_LC|LC_IC1613_TILE
200068681, , , , GO8041_LC|LC_IC1613_TILE
200068682, , , , GO8041_LC|LC_IC1613_TILE
200068683, , , , GO8041_LC|LC_IC1613_TILE
200068684, , , , GO8041_LC|LC_IC1613_TILE
200068685, , , , GO8041_LC|LC_IC1613_TILE
200068686, , , , GO8041_LC|LC_IC1613_TILE
200068687, , , , GO8041_LC|LC_IC1613_TILE
200068688, , , , GO8041_LC|LC_IC1613_TILE
200068689, , , , GO8041_LC|LC_IC1613_TILE
200068690, , , , GO8041_LC|LC_IC1613_TILE
200068691, , , , GO8041_LC|LC_IC1613_TILE
200068692, , , , GO8041_LC|LC_IC1613_TILE
200068693, , , , GO8041_LC|LC_IC1613_TILE
200068694, , , , GO8041_LC|LC_IC1613_TILE
200068695, , , , GO8041_LC|LC_IC1613_TILE
200068696, , , , GO8041_LC|LC_IC1613_TILE
200068697, , , , GO8041_LC|LC_IC1613_TILE
200068698, , , , GO8041_LC|LC_IC1613_TILE
200068699, , , , GO8041_LC|LC_IC1613_TILE
200068700, , , , GO8041_LC|LC_IC1613_TILE
200068701, , , , GO8041_LC|LC_IC1613_TILE
200068702, , , , GO8041_LC|LC_IC1613_TILE
200068703, , , , GO8041_LC|LC_IC1613_TILE
229228820, , , , GO8041_LC
229228821, , , , GO8041_LC
229228822, , , , GO8041_LC
229228823, , , , GO8041_LC
229228824, , , , GO8041_LC
229228825, , , , GO8041_LC
229228826, , , , GO8041_LC
229228827, , , , GO8041_LC
229228828, , , , GO8041_LC
229228829, , , , GO8041_LC
229228830, , , , GO8041_LC
229228831, , , , GO8041_LC
229228832, , , , GO8041_LC
229228833, , , , GO8041_LC
229228834, , , , GO8041_LC
229228835, , , , GO8041_LC
229228836, , , , GO8041_LC
229228837, , , , GO8041_LC
229228838, , , , GO8041_LC
229228839, , , , GO8041_LC
229228840, , , , GO8041_LC
229228841, , , , GO8041_LC
229228842, , , , GO8041_LC
229228843, , , , GO8041_LC
229228844, , , , GO8041_LC
229228845, , , , GO8041_LC
229228846, , , , GO8041_LC
229228847, , , , GO8041_LC
229228848, , , , GO8041_LC
229228849, , , , GO8041_LC
229228850, , , , GO8041_LC
229228851, , , , GO8041_LC
229228852, , , , GO8041_LC
229228853, , , , GO8041_LC
229228854, , , , GO8041_LC
229228855, , , , GO8041_LC
229228856, , , , GO8041_LC
229228857, , , , GO8041_LC
229228858, , , , GO8041_LC
229228859, , , , GO8041_LC
229228860, , , , GO8041_LC
229228861, , , , GO8041_LC
229228862, , , , GO8041_LC
229228863, , , , GO8041_LC
229228864, , , , GO8041_LC
229228865, , , , GO8041_LC
229228866, , , , GO8041_LC
229228867, , , , GO8041_LC
229228868, , , , GO8041_LC
229228869, , , , GO8041_LC