New Insight on the Origin of the Double Red Clump in the Milky Way Bulge

Abstract

The double red clump (RC) observed in the Milky Way bulge is widely interpreted as evidence for an X-shaped structure. We have recently suggested, however, an alternative interpretation based on the multiple population phenomenon, where the bright RC is from helium enhanced second-generation stars (G2), while the faint RC is representing first-generation stars (G1) with normal helium abundance. Here our RC models are constructed in a large parameter space to see the effects of metallicity, age, and helium abundance on the double RC feature. Our models show that the luminosity of RC stars is mainly affected by helium abundance, while the RC color is primarily affected by metallicity. The effect of age is relatively small, unless it is older than 12 Gyr or much younger than 6 Gyr. The observed double RC feature can therefore be reproduced in a relatively large parameter space, once ∆Y between G2 and G1 is assumed to be greater than ∼0.10. We further show that the longitude dependence of the double RC feature at b ≈ −8 • , which was pointed out by Gonzalez et al. (2015) as a potential problem of our model, is well explained in our scenario by a classical bulge embedded in a tilted bar.

Radial Dependence of the Proto-globular Cluster Contribution to the Milky Way Formation

“…The evolutionary population synthesis (EPS) models presented here are constructed under the same assumptions on the input parameters adopted in Chung et al (2016). Readers are referred to Chung et al (2013) and Chung et al (2017) for detailed prescriptions of the EPS models. Figure 1 presents synthetic horizontal branch (HB) stars and corresponding isochrones from the main sequence to the tip of red giant branch stars in (u − g) 0 and (g −r) 0 color-magnitude diagrams (CMDs) at given metallicities and 12 Gyr age.…”

“…Also, the age estimations of Galactic inner halo in the literature suggest around ∼ 11.5 Gyr (e.g., Kalirai 2012;Carollo et al 2016). As presented in Figure 4 of Chung et al (2017), to reproduce bBHB at halo metallicity without assuming He-enhancement, the age of the population would be at least 13 Gyr. Therefore, the age referred to in the literature is not old enough to reproduce BHBs.…”

“…In order to explain the increasing fraction of bBHB stars in the Galactic inner halo 3 , we need to adopt Herich populations usually observed in GCs. Since He-rich stars evolve faster than normal He stars, the He-rich stellar populations originated from GCs could reproduce more bBHB stars at younger ages with a high metallicity of inner halo (Chung et al 2011(Chung et al , 2017. Chung et al (2016) used BSs and BHBs as proxies for FG and SG populations, respectively, and explained that the low BHB-to-BS ratio in the outer halo of the Milky Way is caused by the preferential removal of the normal He FG stars from proto-GCs while He-rich SG stars remain inside of the proto-GCs.…”

Transverse bar/bulge kinematics with Gaia and VVV

“…1) which has been interpreted as an X-shaped bulge. However, the evidence for an X-shaped morphology has been contested (Joo et al (2017)) and a different interpretation is the bulge contains red clump populations with intrinsically different brightnesses (potentially due to differing helium abundances). We now turn to assessing the presence of the X-shaped structure using the new data from Gaia and VVV.…”

“…In response to Gonzalez et al (2015), who argued that the varying weights of the two red clump contributions across the bulge supported the X-shaped scenario, Joo et al (2017) presented a model with a composite bulge composed of two red clump populations with different absolute magnitudes embedded in a bar, which could qualitatively reproduce the data. Both Sanders et al (2019a) and Clarke et al (2019) used proper motions from VVV and Gaia to demonstrate that there are kinematic differences between the two red clump peaks.…”