Investigating a possible dearth of stars with zero angular momentum in the solar neighbourhood.

Abstract

Background. Carlberg & Innanen (1987) first proposed the idea of measuring the solar re- flex velocity (the velocity of the Sun around the galactic centre), Vg,⊙, using the momenta of the stars in the solar neighbourhood. The proposal was a consequence of the work of L. Martinet (Martinet 1974), where he proved that there should be a dearth of stars with zero angular momenta in the solar neighbourhood. The dip, corresponding to the dearth would then be centred around Vg,⊙. Hunt et al. (2016) pursued the idea further using data from the Gaia DR1, RAVE DR5, TGAS catalogues. While their results were encouraging, they were limited by the number of reliable stars that could be used in their model.
Aim. Using the additional data from the Gaia DR3 catalogue, the aim of this report is to measure Vg,⊙ using the idea outlined above.

Results. We start by considering different subsets of our data based on the position of the star relative to the Sun. We use an analytic Lorentzian dip profile to apply to these subsets of data. The parameters are estimated using MCMC. While some results from this analysis do find a dip, they are very sensitive to the subset of data on which the profile is applied. As an alternative, we simulate the orbits of several stars and use the fraction of time they spend near the galactic plane as a proxy model for the dip. The dip profiles generated from these simulations suggest that a dearth of stars should be present. Upon application to the sub-datasets, the results are found to be sensitive to the particular set. We summarise our findings and suggest some ideas that could be pursued to investigate this method hypothesis in the future.

Popular Abstract

The hitchhiker’s guide to the Galactic Center
The fate of anything falling into a black hole, like the one at the centre of our Milky Way, is not what you might think. As always, size matters. While yes, brave humans who dare to wander about such fancy celestial monuments might get squeezed into spaghetti, fortunately not even the most enterprising human will ever encounter one. But what of the fate of stars, like our Sun? While the exact eventuality is slightly hard to predict, it has been proven earlier (Martinet 1974) that most of the stars that pass through the Galactic centre, mostly avoid any spaghett-isation and end up being scattered to chaotic orbits that do not conform to the Galactic plane, where most of the stars orbit.
It also turns out that the stars that end up plunging into the galactic centre have a low circular momentum. Just like slow-moving targets are easy targets for sharpshooters, slow-moving stars are easy targets for the galactic nucleus to swallow. Since most of these slow-moving stars should already have been swallowed by the galactic nucleus, there should be an apparent dearth of these in the vicinity of the Sun. The distribution of the circular velocities of all these stars in some vicinity of the Sun would thus have a dip corresponding to the missing stars that have been swallowed. Intuitively, the dip would be strongest where the stars have 0 circular velocity with respect to the centre.
Over time, as humans have been able to observe farther and better into the emptiness of space, it was imperative to have a convenient static frame of reference in which all the observations were made. Luckily, the Sun was right around the corner to help. The Sun’s proximity to the Earth, size and stationary position (to a good degree of approximation for early land-based observations.) ensured that we had a very accurate map of the surrounding sources. Thus, the magnitude of the circular velocity of the Sun with respect to the Galactic centre is of significant importance. Furthermore, models for the Galaxy also take into account the role dark matter plays in the kinematics of the Milky Way. Thus, accurate knowledge of the velocity would also help to better understand the Galaxy in that the models could be adapted to account for the Sun’s dynamics in turn possibly revealing some details about dark matter.
In my project, I attempt to find the velocity of the Sun using the method described above. Through my labours, I hope to look at the emptiness of space, with a bit of completeness. After all Per Aspera ad Astra. (Through hardship, to the stars.)

• author: Desai, Aayush
• supervisor: Paul McMillan
• organization: Lund Observatory - Has been reorganised; Department of Physics
• course: ASTK02 20222
• year: 2023
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• keywords: Galaxy: disk, Galaxy: fundamental parameters, Galaxy: kinematics and dynamics, Galaxy: nucleus, solar neighborhood, stars: kinematics and dynamics
• publication/series: Lund Observatory Examensarbeten
• report number: 2023-EXA200
• language: English
• id: 9109524
• date added to LUP: 2023-01-31 14:03:22
• date last changed: 2023-08-30 15:20:26