Optical photometric variable stars towards the Galactic H ii region NGC 2282

Abstract

We report here CCD I-band time-series photometry of a young (2−5 Myr) cluster NGC 2282 to identify and understand the variability of premain-sequence (PMS) stars. The I-band photometry, down to ∼ 20.5 mag, enables us to probe the variability towards the lower mass end (∼ 0.1 M ⊙ ) of the PMS stars. From the light curves of 1627 stars, we identified 62 new photometric variable candidates. Their association with the region was established from Hα emission and infrared (IR) excess. Among 62 variables, 30 young variables exhibit Hα emission, near-IR (NIR)/mid-IR (MIR) excess or both, and they are candidate members of the cluster. Out of 62 variables, 41 are periodic variables with the rotation rate ranging from 0.2 to 7 days. The period distribution exhibits a median period at ∼ 1-day as in many young clusters (e.g., NGC 2264, ONC, etc.), but it follows a uni-modal distribution unlike others having bimodality with the slow rotators peaking at ∼ 6−8 days. To investigate the rotation-disk and variability-disk connection, we derived NIR excess from ∆(I−K) and MIR excess from Spitzer [3.6]−[4.5] µm data. No conclusive evidence of slow rotation with the presence of disks around stars and fast rotation for diskless stars is obtained from our periodic variables. A clear increasing trend of the variability amplitude with the IR excess is found for all variables.

Optical photometric variable stars towards Cygnus OB7

“…Figure 11 displays spectral index against rotation period. While the number of stars is too small to provide a good statistical significance, the general form of the distribution is quite similar to what is seen in the ONC (Herbst et al 2002), IC 348 (Nordhagen et al 2006) or NGC 2282 (e.g., Dutta et al 2018a). One CTTS star (ID6506) has the period less than 2 days and another CTTS (ID1564) is relatively slow rotators.…”

“…The identification of variable stars was performed by means of careful inspection of the light curves. First, we estimated RMS (σmag) deviation of relative magnitudes for all the stars (e.g., Dutta et al 2018a). In the Figure 4, the lower envelope represents an expected trend of σmag as a function of stellar brightness, where σmag increases with Iinst magnitudes since SNR decreases accordingly.…”

“…The stars ID1564 and ID1771 also show strong Hα emission (equivalent width > 10 Å) in (r − i)/(r − Hα) CC diagram (Figures are not shown) based on IPHAS photometry (e.g., Barentsen et al 2011;Dutta et al 2018a).…”

“…Since circumstellar disks play a vital role in PMS variability (e.g., Edwards et al 1993;Herbst et al 2002;Nordhagen et al 2006;Dutta et al 2018a), we looked for any possible correlation between NIR excess and rotation. Figure 11 displays spectral index against rotation period.…”

Variable stars in the Sh 2-170 H ii region

“…Lata et al (2012Lata et al ( , 2016 have found a decrease of rotation period of PMS variables with the increase in their masses, but no conclusive result was found for age-period correlation. Dutta et al (2018) have found a correlation between variability amplitude with IR excess in PMS stars, indicating that in the disc bearing stars, the accretion phenomena plays a significant role in their energy output. Unfortunately, we are still not able to construct a complete paradigm for disc evolution and a commonly adopted scenario for early stellar evolution that satisfies all the available observational constraints.…”