Abstract
We demonstrate that failure to properly account for stellar evolution can bias results in determinations of the spatial morphology of Galactic bulge stars, focusing on the question of whether or not the X-shape is more pronounced among the more metal-rich stars than among the metal-poor stars. We argue that this trend, a result recently claimed by three separate groups, may have been overestimated as it is relatively easier to detect a bimodality in the distance distribution function at higher metallicities. This is due to three factors. First, the intrinsic colour of red clump and red giant stars varies with metallicity, at the level d(V - I)RC/d[M/H] ≈ 0.25 mag dex−1, and thus the ratio of red clump to red giant stars within a spectroscopic sample will depend on the photometric selection of any investigation. Secondly, the duration of ascent of the red giant branch goes down and the red clump lifetime goes up as metallicity increases, which has the effect of increasing the ratio of red clump to red giant stars by as much as ∼33 per cent over the range of the bulge metallicity distribution function. Finally, over the same metallicity interval, the effective number of red giant branch bump stars is predicted to increase by ∼200 per cent, and their presence becomes degenerate with the observational parameters of the two red clumps, creating an illusory increase in signal to noise for a second peak in the distance modulus distribution.
Highlights
The apparent magnitude distribution of red clump (RC) stars toward Galactic bulge sightlines that are at least ∼ 5 degrees apart from the plane is bimodal (Nataf et al 2010; McWilliam & Zoccali 2010), an artefact of an excess in the orbital distribution of bulge stars that would appear X-shaped if the Galactic bar were viewed side-on (Ness et al 2012; Li & Shen 2012)
Given that the class of orbits that contributes to this morphology, predominantly trapped by the x1 tree of families, is sharply sensitive to the Galactic gravitational potential (Patsis et al 2002; Athanassoula 2003), and that the kinematics of bulge stars have been shown to be correlated to metallicity (Zoccali et al 2008; Babusiaux et al 2010; Ness et al 2013), mapping how the strength and extent of the X-shape correlates with metallicity could constrain formation and evolution models of the Galaxy
We describe the metallicity-dependence of the predicted parameters for the red giant branch bump (RGBB) and we demonstrate that failure to account for this component of the luminosity function will bias determinations of the distance distribution function
Summary
The apparent magnitude distribution of red clump (RC) stars toward Galactic bulge sightlines that are at least ∼ 5 degrees apart from the plane is bimodal (Nataf et al 2010; McWilliam & Zoccali 2010), an artefact of an excess in the orbital distribution of bulge stars that would appear X-shaped if the Galactic bar were viewed side-on (Ness et al 2012; Li & Shen 2012). It has been suggested that: Stars supporting the X-shape would primarily be disc stars, the latest ones captured into resonance that were in the mid-plane prior to their capture into Such a prediction can only be compared to observations that include both metallicity and kinematic information, requiring an accounting of the systematics thereof. Uttenthaler et al (2012) independently argued, by means of a combination of spectroscopic and photometric data, that stars with metallicity [M/H] −0.20 do not show the split red clump, in contrast to stars with [M/H] −0.20 This demarcation has been recently argued in separate conference presentations by Manuela Zoccali and Alvaro Rojas-Arriagada to be. These are the metallicity-dependence of the colour of the RC (Section 3), the increase in the ratio of RC to red giant (RG) stars with increasing metallicity (Section 4), and the effect of the red giant branch bump (RGBB) (Section 5)
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