PoS(EVN 2014)055 7% ∼ kpc and 3 . 8 http://pos.sissa.it/ ≈ 0 R , indicating that the existence of more dark matter than previously 1 − km s 220 . The latter suggests that the Galaxy’s rotation velocity is higher than the IAU 1 = − 0 ∗ Θ km s 240 ≈ [email protected] 0 Speaker. of SKA’s collecting area would significantlymaser boost astrometry. up the science capability of VLBI including In this paper we presentstructure revealed the with current the status maser ofobtained astrometry. for VLBI To more data maser than parallaxes 100 astrometry sources and andstructure with proper EVN, discuss has motions VERA been the have and been explored Galaxy VLBA, and basedare the on already fundamental determined these Galactic at astrometric a measurements. few % level The accuracy, with Galactic most-likely constants values with Θ standard of expected. Moreover, maser astrometry results providearm, constraints including on their basic locations, structure orientations of and theVLBI dynamics. spiral maser We astrometry also in discuss 2020’s, the and future propose prospect that of a globally combined array comprising ∗ Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. c ⃝ 12th European VLBI Network Symposium and7-10 Users October Meeting, 2014 Cagliari, Italy Mareki Honma Mizusawa VLBI Observatory, National Astronomical ObservatorySOKENDA of (The Japan Graduate University for AdvancedE-mail: Studies) Maser Astrometry with VLBI and Galactic structure PoS(EVN 2014)055 (1.1) Mareki Honma -term was given z . z + λ ] revealed that this term reflects the uncor- sin 19 y + 2 ) describes the instantaneous positon of the north λ y , x cos x = φ ∆ ], which was an additional term necessary to better explain the 6 -term” [ z is the observatory longitude, respectively. The interpretation of the λ is the latitude variation, a vector ( Left: ILS Mizusawa station, the predecessor of the Mizusawa VLBI observatory of NAOJ. Right: φ . They were built for conducting optical observations of stellar transits with an aim to trace ′ ∆ The ILS observations have been continued around 1980, and then both the Mizusawa and Immediately after the start of the ILS observations, Mizusawa observatory made a significant Both Cagliari observatory and Mizusawa observatory have the common origin in the history of 08 ◦ 70 yr later than Kimura’s discovery when Wako[ ∼ Figure 1: ILS Carloforte station in San Pietro island, the predecessor of the Cagliari Observatory of INAF. the Cagliari observatory shifted its focusthe toward beginning geodetic of and the geodynamical studies, thirdVLBI and technology millennium, lately, was at also already available toward for radio preciselyrotation. measuring astronomy station The and positions ILS as VLBI, Mizusawa well observatory as aspart has Earth’s in of been 1980’s NAOJ reformed (National the as AstronomicalExploration Mizusawa Observatory of VLBI of Radio observatory Astrometry) Japan), as as and its a major now facility. it The operates ILS Carloforte VERA observatory (VLBI has become pole, and Here contribution to the ILS: Dr.discovered the Hisashi so-called ” Kimura, theobserved first latitude director variation, i.e., of the ILS Mizusawa observatory rected nutation originating from the liquid core of the Earth. astronomy. They were founded in 1899 asService ones of (ILS). the The six observatories six of the observatoriesto International were: Sardinia), Latitude Tscharudjui Carloforte in in Turkestan, Italyand Mizusawa (located in Gaithersberg Japan, on (Maryland) Ukiah San in (California), Pietro US,39 Cincinnati Island all (Ohio) next of which werethe located variation of at the a Earth’s common polarand motion. northern big At latitude international the of project end in of natural 19th science century, under the the ILS world-wide was collaboration. the most modern Maser Astrometry 1. Preface: historical connection between Cagliari and Mizusawa observatory PoS(EVN 2014)055 ] 2 1/57 (2.1) ∼ ]), and 9 is the speed c Mareki Honma of 1 deg ( SA θ ] and Hachisuka et al. [ O and SiO masers associated 20 2 of 8000 km, as level accuracy is possible (for µ B . t as ! ∆ B µ c 5 SA 3 ]. ∼ θ 12 of ≈ θ θ ∆ ∆ ] obtained S269’s distance to be 5.28 kpc at 4% level. Rygl et al. 4 ]), and thus Galactic-scale astrometry is readily being conducted. 12 is the maximum baseline length, respectively. Note that in the above equation B are described in radian. If one has a baseline of 1 cm, one can achieve t ]). SA ∆ θ c 12 ]). is the measurement error in delay (usually limited by the water vapor fluctuation in the 7 t is the separation angle between the target source and the reference source, and ∆ SA θ θ ∆ For doing VLBI astrometry, one requires sources with high brightness temperature, and the Since early 2000’s, VLBI maser astrometry at kpc-scale has been routinely conducted with Currently there are three major VLBI arrays in the northern hemisphere regularly conducting Astrometric accuracy in phase-referencing VLBI can be determined as Currently this accuracy is unprecedented by optical astrometry such as HIPPARCOS ([ VLBI (Very Long Baseline Interferometry) provides the highest angular resolution amongst with star forming regions and/or laterof type sources stars. which In are addition also to maser interestinghole sources, targets Sgr there for A*, are VLBI micro-QSOs, other astrometry, pulsars types such and as so the on Galaxy [ center black radian), and most commonly observed sources are maser sources such as OH, H VLBA, VERA and EVN. As one of pioneering results, Xu et al. [ astrometry: EVN (European VLBI Network)Baseline in Array) in Europe, US. VERA Also in in thethe Japan, southern only hemisphere, and LBA VLBI VLBA (Long array Baseline (Very in Array) Long operation, inby Australia and LBA is recently ([ there has been Galactic-scale astrometry conducted 3. Recent results in maser astrometry troposphere), and both review, see [ Here of light, GAIA mission is on itscatalog way expected to to be reach released down insources at early at 2020’s. the 5–10 VLBI same kpc already (i.e., accuracy achieved [ a level distance with measurement VLBI, of with the final reported VLBA observations of W3OH withracy. a For parallax VERA, distance Honma of et 1.95–2.05 al. kpc [ at a 2–3% accu- existing telescopes at any wavelengthlution, of VLBI electromagnetic can wave. be usedresolution, With or either to such to precisely a measure obtain high the super-fineFor positions angular the images of latter, reso- celestial of recent objects, progress celestial i.e., in objects high-accuracy phase-referencingrevolutionized astrometry. technique with astrometry for the accuracy, compensating and the highest troposphere now has astrometry with 2. Astrometry with VLBI Maser Astrometry a part of Cagliari AstronomicalTelescope Observatory (SRT) of project. INAF, which is NowadaysVLBI now Japanese observations operating between and the these Italian Sardinia countries Radio radio withobservatories the astronomers are participation still are of actively the conducting collaborating SRT, after test and a hence long these history two over 100 yrs ! PoS(EVN 2014)055 Mareki Honma 4 Distribution of 114 maser sources observed with EVN, VERA and VLBA superposed on the ] also reported maser parallax measurements with EVN, demonstrating that EVN is also a 15 Also prominent in figure 2 is the Galactic rotation (shown as vectors), which appears as a Following the successful initial investigations above, these arrays have been involved in maser , 14 circular rotation in theis first clearly order seen approximation. indetermine the The the difference acceleration position of of due the the directions to dynamical of the center source circular of motions, rotation the and Galaxy based as on well that as one the can mean speed of the astrometry on regular basis:VLBI astrometry, VERA and VLBA has has been beenAlso conducting there continuing kpc-scale have maser been the continuing astrometry operation efforts as oncurrently BeSSeL as VLBI (as project. astrometry of a using the EVN. end dedicated of Thanks 2014), to arrayfor in these total to 114 activities, accurate Galactic parallaxes and maser proper sources motionsthe associated have source been with obtained distribution the on Galactic a plan star-forming viewdisk regions. of and Figure the Galaxy, spiral 2 with shows arms. an artistic100 impression As pc of seen the to in Milky 10 Way’s the kpcfor figure, from tracing the the maser the Sun, sources Galactic and distributedistribution they structures is at cover such still a distances biased as large from toward spiral the fraction athe northern arms of few part VLBI the and of arrays, the Galactic and the Galaxy disk, thus central due the beingfor to bar. observations useful the the of geographic near Note the locations future. of southern that maser Alsomay the sources important need source remain better is important array to issues with obtain longer the baselines parallax and/or results better calibration beyond accuracy. 10 kpc, for which one Figure 2: artistic view of the Milky Waytheir Galaxy. proper The motions. dots The with black arrows spot show the is maser the sources, Galaxy with center, and vectors the showing black spot with a circle is the Sun. [ powerful array for conducting Galactic-scale astrometry. Maser Astrometry PoS(EVN 2014)055 , 1 ]), − kpc 10 seems 16 . 0 km s 0 Θ  14  Mareki Honma 34 . ]) attempted to 8 1 238 = 0 = R 0 Θ 0 8 km/s 16 km/s 14 km/s Θ ] using 18 sources, resulting in  ] refined the Galactic constants   5 11 kpc and 45 . ]), showing that these maser sources 0 18  ). Based on the detailed statistical analyses, 05 . 0 0 8 0.6 kpc0.45 kpc 247 238 0.16 kpc 240 Θ R    = ]) tried to explain the non-circular motion along 0 5 and 16 R 0 . Later Honma et al.[ R 1 , indicating that the mass of the Galaxy is larger than 1 − ] compiled 103 sources to obtain − ]) presented a period-luminosity relation for Miras based src 13 8 N km s 16  ]) and Vlemmings et al. ([ 247 17 = 0 . These results are consistent with each other within the error bars, and the 1 Θ − Reid+(2009) 18Reid+(2014) 8.4 103 8.34 Honma+(2012) 52 8.05 km s 8 kpc and Table 1. Galactic constant determinations based on VLBI astrometry.  6 . 0 240  ] traced the spiral structure with the maser distributions and determined the pitch angles = 4 . 0 13 8 Currently in East Asia, VERA and KVN (Korean VLBI Network) are in joint operation on Beyond the circular rotation, the maser astrometry provides valuable information on the de- Beside the Galactic structure, late-type stars such as super-giant stars and AGB stars are in- Θ = 0 respectively. Most recently Reid et al. [ on VLBI astrometry, which willstars. be an important basis for calibrating the distances to other AGB using 52 sources available in 2012, obtaining are also good targets for VLBItypical astrometry, distance though these of sources kpc are or somewhatstellar less). closer mass-loss, to structure Yet the astrometry of Sun of (a circum-stellarsating these envelope, stars. sources and Recently provide period-luminosity Nakagawa valuable relation et information for al.([ on the pul- 4. Future prospect tailed structure on the Galaxy.et al.[ The spiral arm structure is the main target of such studies: Reid previously expected. This implies that therestrong exists impact more on dark the matter dark in matterto the particle the Galaxy, detection Galactic which experiments constants, brings on-going a one onof can the the Earth. also sources, obtain In which the addition are rotation also curve important shape for and understanding mean the Galaxy’s peculiar structure. motions the Perseus arm with a density-wave-type dynamical model developed by Piñol-Ferrer et al. ([ teresting targets for maser astrometryby with van VLBI. Langevelde Pioneering et works al. have been ([ done with VLBA and last one already reached atpower 2–3 of % the accuracy VLBI in astrometry Galactic (seelarger constant table than determination, 1). the demonstrating Also IAU the important standard is of that 220 the km Galactic s rotation of the spiral arms suchtrace as the Sagittarius, Perseus source and distribution Outer into arms. the these Perseus Also, sources. arms Choi et and Most al.([ they recently, Sakai used et it al. to revise ([ and the revealed kinematic that distance non-circular motions associatedthe with density-wave Perseus model arm with sources a could set of be reasonable described parameters. by Galactic rotation (i.e., the Galactic constants initial results on the Galactic constants are reported by Reid et al. [ Maser Astrometry R PoS(EVN 2014)055 Mareki Honma 7% of SKA. ∼ 6 Major VLBI Arrays in northern hemisphere. Clockwise from top, EVN, VLBA and EAV. Com- For further future, more global collaborations should be essential, and the combination of the In early 2020’s, GAIA will also complete astrometric measurements for billions of stars in more stations should be considered toastrometry. realize better Thus, sensitivity, better a angular combined resolutionthe array and southern of better EVN, hemisphere) VERA is and something VLBA we (and have also to more consider stations even for in our future (e.g., figure 3). The the Galaxy. Since GAIAtraces observes gaseous stars components and in VLBI theboth observes Galaxy), of maser these the two sources optical are (mostlydynamics: and complementary SFR, while radio to the which astrometry each stars are other, are collisionlessand and indeed system, hence thus the important SFRs these for (gaseous two components) thesame populations are understanding Galactic collisional, should potential. the show Therefore, Galaxy for totally testingthe different dynamical both models motions populations of are although the required. they Galaxy, In thea are this information combination sense, of in of Galactic the GAIA astrometry and in VLBI 2020’s will astrometry. be exciting with regular basis as KaVA array (KVNIn and addition, VERA there Array), is consisting a ofin plan seven of Japan, to 20m-class establish Korea antennas. and EAVN (East China.as Asian VLBI well) For Network) in doing by Chinese this, combining VLBI SHAOthese stations Network possible 25m are extensions, and possible we the will stations continue new contributingof operation 65m to VERA, for EAVN telescopes VLBA/BeSSeL next from and 10 (and China. EVN years, others data and With (andwill hopefully also be combination GAIA, of the great European help space for astrometry understanding mission) the details of the Galactic structures. Figure 3: bining these arrays would make a global VLBI network having a collecting area of Maser Astrometry PoS(EVN 2014)055 6 . 5 ≈ 4 / 3 10 Mareki Honma 7% of SKA, and so part of the ∼ 7 . Therefore, if such a powerful array would indeed become available, the whole Galaxy 5 Rygl K., et al., 2010, A&A, 511, 2 Rygl K., et al., 2010, A&A, 539, 79 Sakai N., et al., 2015, PASJ, submitted van Langevelde H. J., et al., 2000, A&A,Vlemmings 357, W., 945 et al., 2003, A&A, 407, 213 Wako Y., 1970, PASJ, 22, 525 Xu Y, et al., Science, 311, 54 Reid M. J. & Homa M., 2014, ARA&A,Reid 52, M. 339 J. et al., 2014, ApJ, 783, 130 Kimura H., 1902, AJ, 27, 107 Krishnan V., et al., 2015, ApJ, submitted Nakagawa A., et al., 2014, PASJ, 66, 101 Perryman M. Am C. et al., 1997, A&A,Piñol-Ferrer, 323 N., L49 Lindblad, P. O., & Fathi, K., 2012,Reid MNRAS, M. 421, J. 1089 et al., 2009, ApJ, 700, 137 Honma M., et al., 2007, PASJ, 59, 889 Honma M., et al., 2012, PASJ, 64, 136 Choi Y. K., et al., 2014„ ApJ, 790, 99 Hachisuka et al. 2006, ApJ, 645, 337 Hachisuka et al. 2015, ApJ, 800, 2 Regarding astrometry, the large collecting area (+ recording with a wider bandwidth) allows ∼ [9] [6] [7] [8] [1] [2] [3] [4] [5] [20] [17] [18] [19] [12] [13] [14] [15] [16] [10] [11] us to observe fainter sourcesenables such us to as conduct those astrometry using in aastrometry the nearby accuracy. calibrator, far which If side will one contribute of in assumes obtaining thethe that better average Galaxy. there separation between is Also, the a target higher sensitivity and sensitivity reference gain should of be an reduced by order a of factor of magnitude, then Maser Astrometry combined array would have ascience collecting for area SKA corresponding phase-I to maythe be science capability done of with VLBI such including high-accuracy a astrometry. combined global array, significantly boosting (assuming the isotropic reference distributions), resultingof in astrometry accuracy better bycould a be factor in the reach of VLBI astrometry. References