Measuring the Sun’s motion with stellar streams Khyati Malhan, Rodrigo Ibata To cite this version: Khyati Malhan, Rodrigo Ibata. Measuring the Sun’s motion with stellar streams. Monthly Notices of the Royal Astronomical Society, Oxford University Press (OUP): Policy P - Oxford Open Option A, 2017, 471 (1), pp.1005-1011. 10.1093/mnras/stx1618. hal-03153292 HAL Id: hal-03153292 https://hal.archives-ouvertes.fr/hal-03153292 Submitted on 27 Feb 2021 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. MNRAS 471, 1005–1011 (2017) doi:10.1093/mnras/stx1618 Advance Access publication 2017 July 4 Measuring the Sun’s motion with stellar streams Khyati Malhan‹ and Rodrigo A. Ibata Universite´ de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550, F-67000 Strasbourg, France Accepted 2017 June 23. Received 2017 June 23; in original form 2017 March 13 ABSTRACT Downloaded from https://academic.oup.com/mnras/article/471/1/1005/3922849 by guest on 27 February 2021 We present a method for measuring the Sun’s motion using the proper motions of Galactic halo star streams. The method relies on the fact that the motion of the stars perpendicular to a stream from a low-mass progenitor is close to zero when viewed from a non-rotating frame at rest with respect to the Galaxy, and that the deviation from zero is due to the reflex motion of the observer. The procedure we implement here has the advantage of being independent of the Galactic mass distribution. We run a suite of simulations to test the algorithm we have developed, and find that we can recover the input solar motion to good accuracy with data of the quality that will soon become available from the ESA/Gaia mission. Key words: Sun: fundamental parameters – stars: kinematics and dynamics – Galaxy: fundamental parameters – Galaxy: halo – Galaxy: structure. toasampleof∼15 000 main-sequence stars from the Hip- 1 INTRODUCTION parcos catalogue. They determined the peculiar velocity to be = ± ± ± −1 In physics, it is always of fundamental importance to know the prop- Vp (10.0 0.36, 5.25 0.62, 7.17 0.38) km s (in the con- erties of the frame from which measurements are being made. Ob- ventional U, V, W directions, respectively). However, Schonrich,¨ servations of astrophysical or cosmological systems, using satellites Binney & Dehnen (2010) caution against this employment of or ground-based telescopes, can be corrected into the Heliocentric Stromberg’s¨ Relation and illustrate, using their chemodynamical frame with ease. However, knowledge of the Sun’s Galactic velocity model of the Galaxy, that the metallicity gradient of the disc popu- V is required to transform any observed Heliocentric velocity into lation causes a systematic shift in the estimation of the kinematics the Galactic frame. This is necessary, for instance, for scientific in- of the Sun. They describe an alternative method to determine the terpretation when studying Galactic dynamics or for correcting the Sun’s velocity with respect to the LSR from the velocity offset that motion of many extragalactic systems (see e.g. Salomon et al. 2016). optimizes their model fit to the observed velocity distribution. Us- ≡ Moreover, the related circular velocity at the solar radius (vcirc ing their chemodynamical evolution model of the Galaxy, described vcirc(R)) also serves as a crucial constraint on the mass models in Schonrich¨ & Binney (2009), they find the Sun’s peculiar motion = . +0.69 . +0.47 . +0.37 −1 of the Milky Way (e.g. Dehnen & Binney 1998a). Therefore, the to be Vp (11 10.75 ,12240.47 ,7250.36 km s ) and estimate determination of V is a crucial task of Galactic astronomy. roughly the systematic uncertainties as (1.0, 2.0, 0.5) km s−1.How- It is important to realize that the Sun’s Galactic velocity V ever, their approach has the disadvantage being based on an exten- needs to be measured with respect to some other reference or tracer. sive modelling of the Milky Way, and hence of being sensitive to A conceptually straightforward way to measure V is to determine the adopted approximations in dynamics and chemistry. the Sun’s motion with respect to a presumed motionless object with Once the Sun’s peculiar velocity is known, one still needs to add respect to the Galaxy. Such measurements are derived from the the velocity of the LSR to obtain the Sun’s velocity with respect observed proper motion of Sgr A∗ (Reid & Brunthaler 2004). But to the Galaxy. It is interesting in this context to examine what it such an approach requires an accurate measurement of R and a is currently possible to measure with respect to nearby tracers. In critical assessment of measurements coming from the dense region a recent contribution, Bobylev (2013) determined the solar Galac- at the Galactic Centre, with its complex dynamical mix of gas, dust, tocentric distance R and Galactic rotational velocity vcirc,as stars and central black hole. modified by Sofue et al. (2011), using data of star-forming re- Alternatively, analyses can be based on local tracers, where one gions and young Cepheids near the solar circle. Based on a sample assumes that the solar motion can be decomposed into a circular of 14 long-period Cepheids with Hipparcos proper motions they = ± = ± −1 motion of the local standard of rest (LSR) plus the so-called peculiar obtained R 7.66 0.36 kpc and vcirc 267 17 km s . = + motion of the Sun with respect to the LSR: V Vcirc Vp. However, with a sample of 18 Cepheids with UCAC4 proper mo- Such a study was presented by Dehnen & Binney (1998b), who tions (among which two were taken from Hipparcos) they found = ± = ± −1 applied the Stromberg¨ relation (Stromberg¨ 1946) in their method R 7.64 0.32 kpc and vcirc 217 11 km s . The dif- ference in the derived vcirc values highlights the difficulty of such measurements, and their sensitivity to the adopted tracers and E-mail: [email protected] data. Masers located in regions of massive star formation have C 2017 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society 1006 K. Malhan and R. A. Ibata also yielded estimates of the LSR motion (254 ± 16 km s−1,Reid et al. 2009; 236 ± 11 km s−1,Bovy,Hogg&Rix2009), though these results are derived from a small number of sources (18) and require knowledge of the velocity lag of the masers with respect to circular motions (Rygl et al. 2010). Here, we will examine the power that streams hold to constrain the solar velocity with respect to the Galaxy. A growing number of stellar streams have been detected in recent years from the Sloan Digital Sky Survey and Pan-STARRS (Odenkirchen et al. 2001; Grillmair & Dionatos 2006; Bernard et al. 2014;Bernard et al. 2016). The most recent contributions come from the AT- LAS survey and surveys with CTIO/DECam (the Atlas stream in Koposov et al. 2014, and the Eridanus and Palomar 15 streams in Downloaded from https://academic.oup.com/mnras/article/471/1/1005/3922849 by guest on 27 February 2021 Myeong et al. 2017). The kinematics of these structures will soon be revealed in the second data release of the Gaia mission survey (Gaia Collaboration et al. 2016). Gaia would possibly also uncover many new low-contrast star streams that are currently below detection limits in star-count surveys. Figure 1. Vector diagram. Red dots represent the positions at successive The key insight about streams that we exploit here is that stream (equal interval) time-steps along a tiny segment of an orbit. vd is the vector v stars move approximately along their orbits, not perpendicular to that measures the path along the orbit. The proper motion vector oμ that v them. That is, the velocity vector of a stream star in the Galaxy’s rest gets measured in observations should lie along d in a non-rotating frame frame must be a tangent vector to the orbit of this extended structure in the Galaxy. But due to the reflex motion of the Sun, the perpendicular vector v ⊥ emerges, causing the deviation of v μ from vector v . at that stellar position. Thus, if we measure any motion perpendicu- r o d lar to the orbital path of the stream at this position, we must reconcile it with the apparent (reflex) motion that emerges due to the motion new alternative method to measure the Sun’s motion that we present of the observer’s frame (from the Sun). Hence, by measuring this in this paper is not correlated with the value of R, it saves us from perpendicular motion vector for the stars in the streams, we can having to analyse observations of densely populated regions in the constrain the Sun’s velocity in the Galaxy. This is not an entirely centre of the Galaxy, it requires only 5D phase-space information new insight. Several studies made in the past that have examined about stream stars (radial velocity constraints are not needed) and the kinematics of stellar streams have had to include (implicitly or does not invoke the need to model the gravitational potential of the explicitly) the solar motion or the circular velocity of the LSR as Milky Way or the stellar populations of the disc.