Near-future surveys promise a dramatic improvement in the number and precision of astrometric, photometric and spectroscopic measurements of stars in the Milky Way's disk. We examine the impact of such surveys on our understanding of the Galaxy by "observing" particle realizations of non-axisymmetric disk distributions orbiting in an axisymmetric halo with appropriate errors and then attempting to recover the underlying potential using a Markov Chain Monte Carlo (MCMC) approach. We demonstrate that the azimuthally averaged gravitational force field in the Galactic plane--and hence, to a lesser extent, the Galactic mass distribution--can be tightly constrained over a large range of radii using a variety of types of surveys so long as the error distribution of the measurements of the parallax, proper motion and radial velocity are well-understood and the disk is surveyed globally. One advantage of our method is that the target stars can be selected non-randomly in real or apparent-magnitude space to ensure just such a global sample without biasing the results. Assuming we can always measure the line-of-sight velocity of a star with at least 1 km/s precision, we demonstrate that the force field can be determined to better than ~1% for Galactocentric radii in the range R=4-20 kpc We conclude that near-future surveys, like SIM Lite, Gaia, and VERA, will provide the first precise mapping of the gravitational force field in the region of the Galactic disk.
Based on the most complete sample of Galactic open star clusters up to 1.8 kpc, we performed statistical analysis of the distribution of open cluster parameters in order to understand the Galactic structure. The geometrical characteristics of a large number of open clusters enable us to determine solar offset and scale height and distribution of reddening material in the Galactic neighbourhood.
We present an analysis of late-O/early-B-powered, parsec-sized bubbles and associated star-formation using 2MASS, GLIMPSE, MIPSGAL and MAGPIS surveys. Three bubbles were selected from the Churchwell et al. (2007) catalog. We confirm that the structure identified in Watson et al. (2008) holds in less energetic bubbles, i.e. a PDR, identified by 8 um emission due to PAHs surrounds hot dust, identified by 24 um emission and ionized gas, identified by 20 cm continuum. We estimate the dynamical age of two bubbles by comparing bubble sizes to numerical models of Hosokawa & Inutsuka (2006). We also i…
The physical properties of galactic winds are of paramount importance for our understanding of galaxy formation. Fortunately, they can be constrained using background quasars passing near star-forming galaxies (SFGs). From the 14 quasar$-$galaxy pairs in our VLT/SINFONI Mgii Program for Line Emitters (SIMPLE) sample, we reobserved the 10 brightest galaxies in H$_α$ with the VLT/SINFONI with 0.7" seeing and the corresponding quasar with the VLT/UVES spectrograph. Applying geometrical arguments to these ten pairs, we find that four are likely probing galactic outflows, three are likely probing e…
It has been suggested that a resonance between a rotating bar and stars in the solar neighbourhood can produce the so called 'Hercules stream'. Recently, a second bar may have been identified in the Galactic centre, the so called 'long bar', which is longer and much flatter than the traditional Galactic bar, and has a similar mass. We looked at the dynamical effects of both bars, separately and together, on orbits of stars integrated backwards from local position and velocities, and a model of the Galactic potential which includes the bars directly. Both bars can produce Hercules like features…
We have applied our method for weighing the Galactic disk using phase-space spirals to the Gaia EDR3 proper motion sample. For stars in distant regions of the Galactic disk, the latitudinal proper motion has a close projection with vertical velocity, such that the phase-space spiral in the plane of vertical position and vertical velocity can be observed without requiring that all stars have available radial velocity information. We divided the Galactic plane into 360 separate data samples, each corresponding to an area cell in the Galactic plane in the distance range of 1.4-3.4 kpc, with an ap…
The origin of the Nuclear Star Cluster in the centre of our Galaxy is still unknown. One possibility is that it formed after the disruption of stellar clusters that spiralled into the Galactic Centre due to dynamical friction. We trace the formation of the Nuclear Star Cluster around the central black hole, using state-of-the-art N-body simulations, and follow the dynamics of the neutron stars born in the clusters. We then estimate the number of Millisecond Pulsars (MSPs) that are released in the Nuclear Star Cluster, during its formation. The assembly and tidal dismemberment of globular clust…
We measure the two-point correlation function of G-dwarf stars within 1-3 kpc of the Sun in multiple lines-of-sight using the Schlesinger et al. G-dwarf sample from the SDSS SEGUE survey. The shapes of the correlation functions along individual SEGUE lines-of-sight depend sensitively on both the stellar-density gradients and the survey geometry. We fit smooth disk galaxy models to our SEGUE clustering measurements, and obtain strong constraints on the thin- and thick-disk components of the Milky Way. Specifically, we constrain the values of the thin- and thick-disk scale heights with 3% and 2%…
We study a set of solar neighborhood ($d < 3$ kpc) stars from Gaia Data Release 2 to determine azimuthal star count differences, i.e., left and right of the line from the Galactic center through the sun - and compare these differences north and south. In this companion paper to Gardner et al. (2020), we delineate our procedures to remove false asymmetries from sampling effects, incompleteness, and/or interloper populations, as this is crucial to tests of axisymmetry. Particularly, we have taken care to make appropriate selections of magnitude, color, in-plane Galactocentric radius and Galactic…
Based on a large spectroscopic sample of $\sim$ 4,300 RR Lyrae stars with metallicity, systemic radial velocity and distance measurements, we present a detailed analysis of the chemical and kinematic properties of the Galactic halo. Using this sample, the metallicity distribution function (MDF) as a function of $r$ and the velocity anisotropy parameter $β$ profiles (for different metallicity populations) are derived for the stellar halo. Both the chemical and kinematic results suggest that the Galactic halo is composed of two distinct parts, the inner halo and outer halo. The cutoff radius ($\…
The precise localization (<1'') of multiple fast radio bursts (FRBs) to z>0.1 galaxies has confirmed that the dispersion measures (DMs) of these enigmatic sources afford a new opportunity to probe the diffuse ionized gas around and in between galaxies. In this manuscript, we examine the signatures of gas in dark matter halos (aka halo gas) on DM observations in current and forthcoming FRB surveys. Combining constraints from observations of the high velocity clouds, OVII absorption, and the DM to the Large Magellanic Cloud with hydrostatic models of halo gas, we estimate that our Galactic halo …
Electromagnetic observations over the last 15 years have yielded a growing appreciation for the importance of supermassive black holes (SMBH) to the evolution of galaxies, and for the intricacies of dynamical interactions in our own Galactic center. Here we show that future low-frequency gravitational wave observations, alone or in combination with electromagnetic data, will open up unique windows to these processes. In particular, gravitational wave detections in the 10^{-5}-10^{-1} Hz range will yield SMBH masses and spins to unprecedented precision and will provide clues to the properties o…
We identify substructures of the Galactic halo using 3,003 type $ab$ RR Lyraes (RRab) with 6D position-velocity information from the SDSS, LAMOST, and Gaia EDR3. Based on the information, we define the separation of any two of the stars in the integrals of motion space and identify substructures by utilizing the friends-of-friends algorithm. We identify members belonging to several known substructures: the Sagittarius stream, the Gaia-Enceladus-Sausage (GES), the Sequoia, and the Helmi streams. In addition to these known substructures, there are three other substructures possibly associated wi…
In diffuse interstellar clouds the chemistry that leads to the formation of the oxygen bearing ions OH+, H2O+, and H3O+ begins with the ionization of atomic hydrogen by cosmic rays, and continues through subsequent hydrogen abstraction reactions involving H2. Given these reaction pathways, the observed abundances of these molecules are useful in constraining both the total cosmic-ray ionization rate of atomic hydrogen (zeta_H) and molecular hydrogen fraction, f(H2). We present observations targeting transitions of OH+, H2O+, and H3O+ made with the Herschel Space Observatory along 20 Galactic s…
The Fermi Bubbles are two giant gamma-ray emitting lobes extending 55$^{\circ}$ above and below the Galactic Center. While the Northern Bubble has been extensively studied in ultraviolet (UV) absorption, little is known about the gas kinematics of the southern Bubble. We use UV absorption-line spectra from the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope to probe the southern Fermi Bubble using a sample of 17 background AGN projected behind or near the Bubble. We measure the incidence of high-velocity clouds (HVC), finding that four out of six sightlines passing through the …
The origin and evolution of neutron-capture elements in our Galaxy is poorly understood. In this work, we have attempted to understand the Galactic s-process nucleosynthesis using barium stars as probes. We have performed high resolution spectroscopic analysis for three barium stars HD 36650, HD 207585 and HD 219116. The analysis of HD 219116 is based on HCT/HESP data at a resolution of 60,000 covering a wavelength region 3530-9970 {\rm Å}. For HD 36650, we have used the FEROS spectrum at a resolution of 48,000 covering the wavelength region 3520-9200 {\rm Å}. For HD 207585, the spectrum from …
The G2 object has recently passed its pericenter passage in our Galactic Center. While the $Br_γ$ emission shows clear signs of tidal interaction, the change in the observed luminosity is only of about a factor of 2, in contention with all previous predictions. We present high resolution simulations performed with the moving mesh code, RICH, together with simple analytical arguments that reproduce the observed $Br_γ$ emission. In our model, G2 is a gas cloud that undergoes tidal disruption in a dilute ambient medium. We find that during pericenter passage, the efficient cooling of the cloud re…
Giant lobes of plasma extend 55 degrees above and below the Galactic Center, glowing in emission from gamma rays (the Fermi Bubbles) to microwaves (the WMAP haze) and polarized radio waves. We use ultraviolet absorption-line spectra from the Hubble Space Telescope to constrain the velocity of the outflowing gas within these regions, targeting the quasar PDS 456 (Galactic coordinates l,b=10.4, +11.2 degrees). This sightline passes through a clear biconical structure seen in hard X-ray and gamma-ray emission near the base of the northern Fermi Bubble. We report two high-velocity metal absorption…
Aims: We aim at measuring mass-loss rates and the luminosities of a statistically large sample of Galactic bulge stars at several galactocentric radii. The sensitivity of previous infrared surveys of the bulge has been rather limited, thus fundamental questions for late stellar evolution, such as the stage at which substantial mass-loss begins on the red giant branch and its dependence on fundamental stellar properties, remain unanswered. We aim at providing evidence and answers to these questions. Methods: To this end, we observed seven 15 times 15 arcmin^2 fields in the nuclear bulge and its…
We present a catalog of 5,290 RR Lyrae stars (RRLs) with metallicities estimated from spectra of the LAMOST Experiment for Galactic Understanding and Exploration (LEGUE) and the Sloan Extension for Galactic Understanding and Exploration (SEGUE) surveys. Nearly 70 per cent of them (3,642 objects) also have systemic radial velocities measured. Given the pulsating nature of RRLs, metallicity estimates are based on spectra of individual exposures, by matching them with the synthetic templates. The systemic radial velocities are measured by fitting the observed velocity as a function of phase assum…
We studied the physical parameters of a sample comprising of all Spitzer/IRS public spectra of Seyfert galaxies in the mid-infrared (5.2-38$μ$m range) under the active galactic nuclei (AGN) unified model. We compare the observed spectra with $\sim10^6$ CLUMPY model spectral energy distributions, which consider a torus composed of dusty clouds. We find a slight difference in the distribution of line-of-sight inclination angle, $i$, requiring larger angles for Seyfert 2 (Sy2) and a broader distribution for Seyfert 1 (Sy1). We found small differences in the torus angular width, $σ$, indicating th…
The detection with CoRoT of solar-like oscillations in nearly 800 red giants in the first 150-days long observational run paves the way for detailed studies of populations of galactic-disk red giants. We investigate which information on the observed population can be recovered by the distribution of the observed seismic constraints: the frequency of maximum oscillation power (nu_max) and the large frequency separation (Deltanu). We propose to use the observed distribution of nu_max and of Deltanu as a tool for investigating the properties of galactic red-giant stars through comparison with sim…
We present the analysis of 12227 type-ab RR Lyrae found among the 200 million public lightcurves in the Catalina Surveys Data Release 1 (CSDR1). These stars span the largest volume of the Milky Way ever surveyed with RR Lyrae, covering ~20,000 square degrees of the sky (0 < RA < 360, -22 < Dec < 65 deg) to heliocentric distances of up to 60kpc. Each of the RR Lyrae are observed between 60 and 419 times over a six-year period. Using period finding and Fourier fitting techniques we determine periods and apparent magnitudes for each source. We find that the periods at generally accurate to sigma …
We present multi-scale and multi-wavelength observations of the Galactic HII region S305, which is excited by massive O8.5V and O9.5V stars. Infrared images reveal an extended sphere-like shell (extension ~7.5 pc; at T_d = 17.5-27 K) enclosing the S305 HII region (size ~5.5 pc; age ~1.7 Myr). The extended structure observed in the Herschel temperature map indicates that the molecular environment of S305 is heated by the massive O-type stars. Regularly spaced molecular condensations and dust clumps are investigated toward the edges of the infrared shell, where the PAH and H_2 emission is also o…
We performed a detailed study of the extended cool gas, traced by MgII absorption [$W_r(2796)\geq0.3$~Å], surrounding 14 narrow-line active galactic nuclei (AGNs) at 0.12<z<0.22 using background quasar sight-lines. The background quasars probe the AGNs at projected distances of $60\leq D\leq265$~kpc. We find that, between $100\leq D\leq200$~kpc, AGNs appear to have lower MgII gas covering fractions (0.09$^{+0.18}_{-0.08}$) than quasars (0.47$^{+0.16}_{-0.15}$) and possibly lower than in active field galaxies (0.25$^{+0.11}_{-0.09}$). We do not find a statistically significant azimuthal angle d…
Comparison of ISM absorption in the UV spectrum of LS 4825, a B1 Ib-II star d=21+/-5 kpc from the Sun toward l = 1.67 deg and b = -6.63 deg, with ISM absorption toward an aligned foreground star at d < 7.0+/-1.7 kpc, allows us to isolate and study gas associated with the Milky Way nuclear wind. Spectra from the Space Telescope Imaging Spectrograph (STIS) show low ion absorption out to d < 7 kpc ( e.g., O I, C II, Mg II, Si II, Fe II, S II) only between 0 and 40 km/s, while absorption at d > 7 kpc, ~1 kpc below the galactic plane, is complex and spans -290 to + 94 km/ s. The intermediate and hi…
This paper explores the possibility of determining the spin of the supermassive black hole (SMBH) in Sgr A*, by using secondary images of stars orbiting the SMBH. The photons propagate close to the SMBH and their trajectories probe the space time in a region where the spin of the SMBH is important. We find the appearance of spikes in the secondary image, which depends on the angular momentum and spin axis of the SMBH and study the specific case of the star S2 in detail. The spikes has a magnitude of $\sim 29$ in the K-band and the required angular resolution is of order 15-20 $μ$as. The combin…
We describe the kinematics of circumgalactic gas near the galactic plane, combining new measurements of galaxy rotation curves and spectroscopy of background quasars. The sightlines pass within 19--93 kpc of the target galaxy and generally detect Mg II absorption. The Mg II Doppler shifts have the same sign as the galactic rotation, so the cold gas co-rotates with the galaxy. Because the absorption spans a broader velocity range than disk rotation can explain, we explore simple models for the circumgalactic kinematics. Gas spiraling inwards (near the disk plane) offers a successful description…
The ultraviolet (UV) polarization spectrum of nearby active galactic nuclei (AGN) is poorly known. The Wisconsin Ultraviolet Photo-Polarimeter Experiment and a handful of instruments on board the Hubble Space Telescope were able to probe the near- and mid-UV polarization of nearby AGN, but the far-UV band (from 1200 angs down to the Lyman limit at 912 angs) remains completely uncharted. In addition, the linewidth resolution of previous observations was at best 1.89 angs. Such a resolution is not sufficient to probe in detail quantum mechanical effects, synchrotron and cyclotron processes, scat…
The Planck Early Release Compact Source Catalogue (ERCSC) includes nine lists of highly reliable sources, individually extracted at each of the nine Planck frequency channels. To facilitate the study of the Planck sources, especially their spectral behaviour across the radio/infrared frequencies, we provide a "bandmerged" catalogue of the ERCSC sources. This catalogue consists of 15191 entries, with 79 sources detected in all nine frequency channels of Planck and 6818 sources detected in only one channel. We describe the bandmerging algorithm, including the various steps used to disentangle so…
The Galactic center (GC) holds the closest-to-Earth supermassive black hole (SMBH), which makes it the best laboratory to study the close environment of extremely massive compact objects. Polarimetry is sensitive to geometry of the source, which makes it a particularly suitable technique to probe the medium surrounding the GC SMBH. The detection of hard X-ray spectra and prominent iron K$α$ fluorescence features coincident with localized gas clouds (e.g. Sgr~B2, Sgr~C) is known for nearly twenty years now and is commonly associated with a past outburst of the SMBH whose radiation is reprocessi…
We determined Faraday rotation measures (RMs) towards 137 pulsars in the northern sky, using Low-Frequency Array (LOFAR) observations at 110-190 MHz. This low-frequency RM catalogue, the largest to date, improves the precision of existing RM measurements on average by a factor of 20 - due to the low frequency and wide bandwidth of the data, aided by the RM synthesis method. We report RMs towards 25 pulsars for the first time. The RMs were corrected for ionospheric Faraday rotation to increase the accuracy of our catalogue to approximately 0.1 rad m$^{\rm -2}$. The ionospheric RM correction is …
Classical double-mode pulsators (RR Lyrae stars and delta Cepheids) are important for their simultaneous pulsation in low-order radial modes. This enables us to put stringent constraints on their physical parameters. We use 30 bright galactic double-mode RR~Lyrae (RRd) stars to estimate their luminosities and compare them with those derived from the parallaxes of the recent data release (EDR3) of the Gaia survey. We employ pulsation and evolutionary models, together with observationally determined effective temperatures to derive the basic stellar parameters. Excluding 6 outlying stars (e.g., …
Flares in X-ray and near infrared are observed above the quiescent emission of the supermassive black hole (SBH) in the Galactic Centre (GC) at a rate of approximately once per day. One proposed energy source for these flares is the tidal disruption of planetesimals with radius $\gtrsim 10$ km passing within $\sim$ 1 AU of the SBH. Very little is known about the formation and evolution of planetesimals in galactic nuclei such as the GC, making predictions for flaring event rates uncertain. We explore two scenarios for the formation of planetesimals in the GC: (1) in a large-scale cloud bound t…
General Relativity (GR) has been successfully tested mainly at Solar system scales; however, galaxy-scale tests have become popular in the last few decades. In this work, we investigate the $η_\text{PPN}$ parameter, which is commonly defined by the ratio of two scalar potentials that appears in the cosmological linearly perturbed metric. Under the assumption of GR and a vanish anisotropic stress tensor, $η_\text{PPN}= 1$. Using ALMA, HST, and VLT/MUSE data, we combine mass measurements, using gravitational lensing and galactic dynamics, for the SDP.81 lens galaxy ($z = 0.299$) to constrain $η_…
We construct axisymmetric mass models for dwarf spheroidal (dSph) galaxies in the Milky Way to obtain plausible limits on the non-spherical structure of their dark halos. This is motivated by the fact that the observed luminous parts of the dSphs are actually non-spherical and Cold Dark Matter (CDM) models predict non-spherical virialized dark halos. Our models consider velocity anisotropy of stars $\bar{v^2_R} / \bar{v^2_ϕ}$, which can vary with the adopted cylindrical coordinates under the assumption $\bar{v^2_z}=\bar{v^2_R}$ for simplicity, and also include an inclination of the system as a…
We present a first application of the recently proposed LITMUS test for magnetic helicity, as well as a thorough study of its applicability under different circumstances. In order to apply this test to the galactic magnetic field, the newly developed critical filter formalism is used to produce an all-sky map of the Faraday depth. The test does not detect helicity in the galactic magnetic field. To understand the significance of this finding, we made an applicability study, showing that a definite conclusion about the absence of magnetic helicity in the galactic field has not yet been reached.…
Paleo-detectors are a proposed experimental technique to search for dark matter by reading out the damage tracks caused by nuclear recoils in small samples of natural minerals. Unlike a conventional real-time direct detection experiment, paleo-detectors have been accumulating these tracks for up to a billion years. These long integration times offer a unique possibility: by reading out paleo-detectors of different ages, one can explore the time-variation of signals on megayear to gigayear timescales. We investigate two examples of dark matter substructure that could give rise to such time-vary…
Probing sub-galactic mass structure with the power spectrum of surface-brightness anomalies in high-resolution observations of galaxy-galaxy strong gravitational lenses. II.Observational constraints on the sub-galactic matter power spectrum
Temporal scatter-broadening can seriously affect our ability to find pulsars orbiting the central mass in our Galaxy. Many of these invaluable probes of geometry around the black hole are expected, but none have been found in close orbits so far, possibly as result of strong scattering. The magnetar PSR J1745-2900 discovered in 2013 at a separation of < 3 arcsec is not the optimal type of pulsar for studies of general relativity, but it can be used to investigate the scattering properties so that search strategies can be adapted accordingly. This contribution presents an observation of this ma…
We present a study of the M83 cluster population, covering the disc of the galaxy between radii of 0.45 and 4.5 kpc. We aim to probe the properties of the cluster population as a function of distance from the galactic centre. We observe a net decline in cluster formation efficiency ($Γ$, i.e. the amount of star formation happening in bound clusters) from about 26 % in the inner region to 8 % in the outer part of the galaxy. The recovered $Γ$ values within different regions of M83 follow the same $Γ$ versus star formation rate density relation observed for entire galaxies. We also probe the ini…
Recent UV-optical surveys have been successful in finding tidal disruption events (TDEs), in which a star is tidally disrupted by a supermassive black hole (BH). These TDEs release a huge amount of radiation energy ~ 10^51-52 erg into the circum-nuclear medium. If the medium is dusty, most of the radiation energy will be absorbed by dust grains within ~ 1 pc from the BH and re-radiated in the infrared. We calculate the dust emission lightcurve from a 1-D radiative transfer model, taking into account the time-dependent heating, cooling and sublimation of dust grains. We show that the dust emiss…
Recent observations with the MeerKAT radio telescope reveal a unique population of faint non-thermal filaments pervading the central molecular zone (CMZ). Some of those filaments are organized into groups of almost parallel filaments, seemingly sorted by their length, so that their morphology resembles a harp with radio emitting "strings". We argue that the synchrotron emitting GeV electrons of these radio harps have been consecutively injected by the same source (a massive star or pulsar) into spatially intermittent magnetic fiber bundles within a magnetic flux tube or via time-dependent inje…
Recent observations have revealed interstellar features that apparently connect energetic activity in the central region of our Galaxy to its halo. The nature of these features, however, remains largely uncertain. We present a Chandra mapping of the central 2x4 square degree field of the Galaxy, revealing a complex of X-ray-emitting threads plus plume-like structures emerging from the Galactic center (GC). This mapping shows that the northern plume or fountain is offset from a well-known radio lobe (or the GCL), which however may represent a foreground HII region, and that the southern plume i…
Pulsars around the Massive Black Hole (MBH) in the Galactic Center (GC) are expected to be revealed by the incoming facilities (e.g., the Square Kilometre Array). Under a full relativistic framework with the pulsar approximated as a test particle, we investigate the constraints on the spinning of the MBH by monitoring the timing of surrounding pulsars. For GC pulsars orbiting closely around the MBH (e.g., $\lesssim1000$AU), we find that full relativistic treatment in modeling accurately their timing signals can be necessary, as the relativistic signals are orders of magnitude larger than the t…
Probing sub-galactic mass structure with the power spectrum of surface-brightness anomalies in high-resolution observations of galaxy-galaxy strong gravitational lenses. I. Power-spectrum measurement and feasibility study
The rotational kinematic Sunyaev-Zeldovich (rkSZ) signal, imprinted on the cosmic microwave background (CMB) by the gaseous halos (spinning "atmospheres") of foreground galaxies, would be a novel probe of galaxy formation. Although the signal is too weak to detect in individual galaxies, we analyze the feasibility of its statistical detection via stacking CMB data on many galaxies for which the spin orientation can be estimated spectroscopically. We use an "optimistic" model, in which fully ionized atmospheres contain the cosmic baryon fraction and spin at the halo's circular velocity $v_{\rm …
We present the analysis of the XMM-Newton data of the Circum-Galactic Medium of MASsive Spirals (CGM-MASS) sample of six extremely massive spiral galaxies in the local Universe. All the CGM-MASS galaxies have diffuse X-ray emission from hot gas detected above the background extending $\sim(30-100)\rm~kpc$ from the galactic center. This doubles the existing detection of such extended hot CGM around massive spiral galaxies. The radial soft X-ray intensity profile of hot gas can be fitted with a $β$-function with the slope typically in the range of $β=0.35-0.55$. This range, as well as those $β$ …
The rich absorption spectra of Sun-like stars are enticing probes for variations in the fine-structure constant, $α$, which gauges the strength of electromagnetism. While individual line wavelengths are sensitive to $α$, they are also sensitive to physical processes in the stellar atmospheres, which has precluded their use so far. Here we demonstrate a new, differential approach using solar twins: velocity separations between close pairs of transitions are compared across stars with very similar physical properties, strongly suppressing astrophysical and instrumental systematic errors. We util…
