XXXth General Assembly of the International Astronomical Union, Vienna, August 20-31, 2018

Astrometric observation of the Galactic LPVs with VERA; Miras and OH/IR stars
Akiharu Nakagawa(1), Tomoharu Kurayama(2), Takahiro Nagayama(1), Toshihiro Omodaka(1), Tomoaki Oyama(3), Takumi Nagayama(3), Mareki Honma(3)（1）Kagoshima university, （2）Teikyo University of Science, （3）NAOJ Mizusawa VLBI Observatory
nakagawa@sci.kagoshima-u.ac.jp

Introduction

Results: Single dish monitoring at 43 GHz (SiO maser)

We present studies of Long Period Variables (LPVs) in out Galaxy based on
astrometric VLBI observations of H2O/SiO maser with VERA. The Galactic
Miras and OH/IR stars are main targets. For Miras, we present distribution of
the Galactic LPVs on Period-Mk plane. We present parallax measurement ofan extreme-OH/IR star (Period > 1000 day) obtained with VERA 43GHz
observations. Annual parallaxes from VLBI and Gaia DR2 are compared.

Results of single-dish monitoring of two extreme-OH/IR stars
OH127.8+0.0 and NSV25875 are shown in figure 7. We want toconfirm availability of SiO maser variation to derive pulsationperiod of the extreme-OH/IR stars. Using these monitoring
results, we determine start date of monitoring VLBI
observations with VERA.

Mira variable The LPVs are 1 - 8 Msun AGB

We selected samples of extreme-OH/IR stars from,(1) Database of Circumstellar Masers[*], and(2) Nançay 1612MHz monitoring of OH/IR star[**].

stars pulsating with typical period range of
100 - 1000 day. They are on the late stage of
their life time, and show high mass loss ratio(10^-7 Msun/yr) before they evolve toplanetary nebulae. In figure1, LPVs in theLMC show some period-luminosity relations
Fig 1: Sequences of variable stars(PLR), and the PLR is used as distancefound in LMC. (Ita et al. 2004)
indicator for Miras in our Galaxy.
But metallicity is different between the LMC and our Galaxy, it is also
important to explore PLR of Miras in MWG. Since 2003, we have been
observing dozens of Miras in the Galaxy. The latest Galactic Mira’s PLR
determined from our study is Mk = －.52 logP+1.09 (±0.14) (Nakagawaet al. 2016). New results will be added here.
OH/IR star There are some OH/IR stars showing quite long period longer

than 1000 day (extreme-OH/IR). They are thought to have initial mass of~4 Msun and ages of ~10^8 yr. Recent studies predict galactic spiral arms
bifurcating/merging in time scale of 10^8 yr. So, the extreme-OH/IR stars
can become a new probe to survey spiral arms. We started 43GHz VLBI
observations of two Extreme-OH/IR stars, OH127.8+0.0 and NSV25875
from Nov. 2017. Since evolutional relation between Miras and OH/IR stars
is still an open question, sequential studies of LPVs along wide period axis
are also crucial.
・Recent simulation studies of spiral arms
→ “Density wave” vs "Non-steady spiral arms"
・Extreme-OH/IR stars (P > 1000 d)→ M： ~ 4 Msun （Feast 2008）→ Age : 108 - 109 yr
・Extreme-OH/IR star can be good sample to inspect
kinematics of stars with ages of 108 - 109 yr.
It can offer new data to the study of spiral arms.

NSV25875
OH127.8+0.0 SiO (v=1)

OH127.8+0.0

Fig 7:Spectr and time variation of SiO
maser in OH127.8+0.0 and NSV25875.

NSV25875 SiO (v=2)

OH127.8+0.0 SiO (v=2)

Ta[K]×19.6 ⇒ Flux Density [Jy][*] http://www.hs.uni-hamburg.de/~st2b102/maserdb/[**] http://www.hs.uni-hamburg.de/DE/Ins/Per/Engels/engels/nrt_monitoring/index.html

Results: Astrometry of Galactic LPVs with VLBI method
Table 1: Results of astrometric observation from VLBI and Gaia DR2

Table 1 shows all parallaxes measured with astrometric VLBI(VERA and VLBA). Gaia DR2 parallaxes are also presented.
Some semiregulars, red giants, and OH/IR stars are also included.
The K-band absolute magnitudes (MK) errors are based on their
VLBI distance errors. Figure 8 shows MK - log P diagram. Filledand open circles indicate Miras and other types of variables. We
find consistency of PLRs between LMC and our Galaxy, butsome stars show fainter magnitudes than expected from the PLR
previously obtained. Though the discrepancy should carefully beinvestigated, this can possibly indicate different properties ofMiras in the LMC and our Galaxy.

・ Period=1000 days
→ M=~4M◎ （Feast 2008）
89・ Age : 10 - 10 yr

Fig 2: HR diagram.

Fig 8: Absolute K-band magnitude (Mk) of Galactic LPVs obtained fromapparent magnitude (mk) and VLBI parallaxes. Three solid lines show
sequences in literatures. Filled and open circles are results of Mira and othertype of variables.

OH/IR stars with P > 1000 day（Kinematic distance & Phase-lag method）

Results: Astrometry of SiO maser in NSV25875 and H2O maser in OH231.8+4.2
Figure 9 is the latest results of a phase referencing VLBI observation withVERA. A target source “NSV25875” is one of the extreme-OH/IR stars. We
can compare the VLBI parallax with distance estimations from “Phase-lag
method” and “Kinematic distance”.

・Root of the Scut-Cent Arm・Following the Perseus Arm

OH127.8+0.0 01h33m51.21s +62d26'53.2"
OH104.9+2.4 22h19m27.48s +59d51'21.7"

VERA observation targets

・Phase referencing analysis ⇒ Annual parallax
・NSV25875 and J2231+5922
・Pulsation period =1748 dayMethod(Engels et al. 2015)
VERA parallax
・Annual parallax = 0.38±0.13 mas
Phase-lag method⇒ Distance = 2.60±0.85 kpc
Kinematic distance・μ_RA = －0.97±0.55 mas/yr
・μ_DEC = －1.06±0.31 mas/yr

We also conduct 22GHzVLBI
astrometric observations. It canreveal internal motions of masers
around central star.
Figure10 shows distribution ofH2O maser in OH231.8+4.2
obtained with VERA. Fitting with
3D-cone model gives opening
angle of ~58±1 degree.

Fig 3: Distribution of extreme-OH/IR stars. Distances are estimated from two
methods,
and Phase-lag
method.
There aresome source which
OH/IRKinematicstars asdistancenew probesof theGalacticdynamics
seems to trace spiral arms. Two sources are being observed with VERA at 43 GHz.

SED of Mira and OH/IR star Miras and OH/IR star often represent H2O, SiO,and OH masers, and they are good target of VLBI astrometric observation.
Because of heavy dust and molecular shell around its central star, infraredemission is prominent in OH/IR stars. Since the Galactic LPVs with longer
period tend to be fainter in visible band, VLBI astrometry play a promisingand complementary role even in Gaia era.
Mira variable

OH/IR starDistance [kpc]2.60±0.85
2.1±0.42
2.50±1.18

Fig 10: Angular distribution and internal motions of H2O masers in a PPN“OH231.8+4.2”, with a 3D-cone model. (Ooyama et al. in prep.)(a)

SiO maser (v=2)
Peak 1.97 Jy/beam
S/N ~ 10(c)

(b)

(d)

Fig 9: (a)43GHz SiO v=2 maser spot of an OH/IR star NSV25875 on 1 Nov 2017.
(b)Sky plane motion of the maser spot from 1 Nov., 2017 to 4 May, 2108. (c)RAmotion of the spot. Horizontal axis is days from 1 Jan., 2017. (d)DEC motion.

Annual parallax of Galactic LPVs from VLBI and Gaia DR2R Cas

NSV25875 (OH104.9+2.4)

namePeriod UVRIJHKB[day] [mag] [mag] [mag] [mag] [mag] [mag] [mag] [mag]1380/
OH127.8+0.0
16.7 11.2 7.1
1994
NSV25875 1535
16.4 10.7 6.9

OH127.8+0.0
F12[Jy]F25[Jy]

F60 F100[Jy] [Jy]289456

194123229

90.7 35.0

50.2

Fig 4: SED of OH/IR stars and Miras

On 25 April 2018, parallaxes of more than 1.3×109
sources were released in Gaia DR2. Since large part ofVLBI parallaxes are determined for star forming regions
deeply obscured by heavy dust and molecular cloud, it isdifficult to find counterpart of the VLBI targets in theGaia DR2. On the other hand, LPVs can easily be
identified in the Gaia DR2. They are good samples forcomparison of parallax from VLBI and Gaia. In table 1,parallax from Gaia DR2 is also given. We introduce a
residual parameter(a) All sources+0.94 σ(b) Mira variables+1.76 σObservation : Single-dish and VLBI・Single-dish monitoring at VERA IRIKI 20mtelescope, to grasp variability of target.
・Freq: 22 GHz (H2O), and 43GHz (SiO)

Fig5: VERAIRIKI station

Fig 6:VERA array

・Astrometric VLBI with VERA・Freq: 22 GHz (H2O), and 43GHz (SiO)
・Duration: 1-2 yr with 1month interval・Technique: Phase referencing

to evaluate difference between
VLBI and Gaia. ΠVLBI(Gaia)and σVLBI(Gaia) are parallax
and its formal error obtained
from VLBI (Gaia).
Fig 11: Pulsation period vs parallax
errors from VLBI (●) and Gaia (〇).
Pulsation period range 100 to 1400 days.

Fig 12: Annual parallaxes determined from VLBI (horizontalaxis) and Gaia (vertical axis) in logarithmic scale.

Fig 13:Histogram of the residual.
Gaia and fitted Gaussian function.
(a)Histogram for all sources (Mira,SR, OH/IR). (b)Histogram for Miravariables.