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Register a new userThree classical Cepheid variable stars in the nuclear bulge of the Milky Way
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The nuclear bulge is a region with a radius of about 200 parsecs around the centre of the Milky Way. It contains stars with ages ranging from a few million years to over a billion years, yet its star-formation history and the triggering process for star formation remain to be resolved. Recently, episodic star formation, powered by changes in the gas content, has been suggested. Classical Cepheid variable stars have pulsation periods that decrease with increasing age, so it is possible to probe the star-formation history on the basis of the distribution of their periods. Here we report the presence of three classical Cepheids in the nuclear bulge with pulsation periods of approximately 20 days, within 40 parsecs (projected distance) of the central black hole. No Cepheids with longer or shorter periods were found. We infer that there was a period about 25 million years ago, and possibly lasting until recently, in which star formation increased relative to the period of 30-70 million years ago.
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The Milky Way is a barred spiral galaxy, with physical properties inferred from various tracers informed by the extrapolation of structures seen in other galaxies. However, the distances of these tracers are measured indirectly and are model-dependent. We constructed a map of the Milky Way in three-dimensions based on the positions and distances of thousands of classical Cepheid variable stars. This map shows the structure of our Galaxys young stellar population, and allows us to constrain the warped shape of the Milky Ways disk. A simple model of star formation in the spiral arms reproduces the observed distribution of Cepheids.
We report on the discovery of the most distant Milky Way (MW) stars known to date: ULAS J001535.72$+$015549.6 and ULAS J074417.48$+$253233.0. These stars were selected as M giant candidates based on their infrared and optical colors and lack of proper motions. We spectroscopically confirmed them as outer halo giants using the MMT/Red Channel spectrograph. Both stars have large estimated distances, with ULAS J001535.72$+$015549.6 at $274 pm 74$ kpc and ULAS J074417.48$+$253233.0 at 238 $pm$ 64 kpc, making them the first MW stars discovered beyond 200 kpc. ULAS J001535.72$+$015549.6 and ULAS J074417.48$+$253233.0 are both moving away from the Galactic center at $52 pm 10$ km s$^{-1}$ and $24 pm 10$ km s$^{-1}$, respectively. Using their distances and kinematics, we considered possible origins such as: tidal stripping from a dwarf galaxy, ejection from the MWs disk, or membership in an undetected dwarf galaxy. These M giants, along with two inner halo giants that were also confirmed during this campaign, are the first to map largely unexplored regions of our Galaxys outer halo.
I review the literature covering the issue of interstellar extinction toward the Milky Way bulge, with emphasis placed on findings from planetary nebulae, RR Lyrae, and red clump stars. I also report on observations from HI gas and globular clusters. I show that there has been substantial progress in this field in recent decades, most particularly from red clump stars. The spatial coverage of extinction maps has increased by a factor $sim 100 times$ in the past twenty years, and the total-to-selective extinction ratios reported have shifted by $sim$20-25%, indicative of the improved accuracy and separately, of a steeper-than-standard extinction curve. Problems remain in modelling differential extinction, explaining anomalies involving the planetary nebulae, and understanding the difference between bulge extinction coefficients and standard literature values.
We study the line widths in the [ion{O}{3}]$lambda$5007 and H$alpha$ lines for two groups of planetary nebulae in the Milky Way bulge based upon spectroscopy obtained at the Observatorio Astronomico Nacional in the Sierra San Pedro Martir (OAN-SPM) using the Manchester Echelle Spectrograph. The first sample includes objects early in their evolution, having high H$beta$ luminosities, but [ion{O}{3}]$lambda 5007/mathrm Hbeta < 3$. The second sample comprises objects late in their evolution, with ion{He}{2} $lambda 4686/mathrm Hbeta > 0.5$. These planetary nebulae represent evolutionary phases preceeding and following those of the objects studied by Richer et al. (2008). Our sample of planetary nebulae with weak [ion{O}{3}]$lambda$5007 has a line width distribution similar to that of the expansion velocities of the envelopes of AGB stars, and shifted to systematically lower values as compared to the less evolved objects studied by Richer et al. (2008). The sample with strong ion{He}{2} $lambda 4686$ has a line width distribution indistinguishable from that of the more evolved objects from Richer et al. (2008), but a distribution in angular size that is systematically larger and so they are clearly more evolved. These data and those of Richer et al. (2008) form a homogeneous sample from a single Galactic population of planetary nebulae, from the earliest evolutionary stages until the cessation of nuclear burning in the central star. They confirm the long-standing predictions of hydrodynamical models of planetary nebulae, where the kinematics of the nebular shell are driven by the evolution of the central star.
The stellar population of the Milky Way bulge is thoroughly studied, with a plethora of measurements from virtually the full suite of instruments available to astronomers. It is thus perhaps surprising that alongside well-established results lies some substantial uncertainty in its star-formation history. Cosmological models predict the bulge to host the Galaxys oldest stars for [Fe/H]$lesssim -1$, and this is demonstrated by RR Lyrae stars and globular cluster observations. There is consensus that bulge stars with [Fe/H]$lesssim0$ are older than $t approx10$ Gyr. However, at super-solar metallicity, there is a substantial unresolved discrepancy. Data from spectroscopic measurements of the main-sequence turnoff and subgiant branch, the abundances of asymptotic giant branch stars, the period distribution of Mira variables, the chemistry and central-star masses of planetary nebulae, all suggest a substantial intermediate-age population ($t approx 3$ Gyr). This is in conflict with predictions from cosmologically-motivated chemical evolution models and photometric studies of the main-sequence turnoff region, which both suggest virtually no stars younger than $t approx 8$ Gyr. A possible resolution to this conflict is enhanced helium-enrichment, as this would shift nearly all of the age estimates in the direction of decreasing discrepancy. Enhanced helium-enrichment is also arguably suggested by measurements of the red giant branch bump and the R-parameter.
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