The neutron star born in the Antlia supernova remnant

Abstract

Among all known young nearby neutron stars, we search for the neutron star that was born in the same supernova event that formed the Antlia supernova remnant (SNR). We also look for a runaway star that could have been the former companion to the neutron star (if it exists) and then got ejected due to the same supernova.

We find the pulsar PSR J0630−2834 to be the best candidate for a common origin with the Antlia SNR. In that scenario, the SNR is ≈1.2 Myr old and is presently located at a distance of ≈138 pc. We consider the runaway star HIP 47155 a former companion candidate to PSR J0630−2834. The encounter time and place is consistent with both stars being ejected from the Antlia SNR. We measured the radial velocity of HIP 47155 as 32.42 ± 0.70 km s⁻¹.

1 INTRODUCTION

The Antlia supernova remnant (SNR), located at (l, b) = (276| $_{.}^{\circ}$|5, 19°), was first discovered by McCullough, Fields & Pavlidou (2002) in X-ray observations. It was later confirmed as an SNR in the ultraviolet (Shinn et al. 2007). Its observability in the UV range as well as its large projected diameter of ≈24 ° suggests small distance (up to a few hundred pc). McCullough et al. (2002) estimated the distance to the Antlia SNR as d_A ≲ 500 pc with a preference for smaller distances (≈100 pc). They assessed an SNR age of ≈2 Myr supported by their suggestion that the pulsar PSR J0953+0755 and the Antlia SNR share a common origin. Given the detection of ⁶⁰Fe in the Earth's crust of nearly ≈2 Myr old (Knie et al. 2004; Fitoussi et al. 2008), McCullough et al. (2002) also considered whether this could have been formed in that supernova. However, McCullough et al. (2002) only considered eight nearby pulsars listed by Hoogerwerf, de Bruijne & de Zeeuw (2001). Therefore, it is worthwhile to re-visit this issue.

Due to large uncertainties in the distances of neutron stars (NSs) and their (in most cases) unknown radial velocities, multiple possible birth places (young associations and clusters, isolated SNRs) can usually be associated (Tetzlaff et al. 2010, 2011). Therefore, further indicators are needed to decide on a particular birth place. A promising indicator is the identification of a possible former companion that is now a so-called runaway star (Blaauw 1961). These are typically fast-moving young single stars that may show signs of former binary evolution such as high helium abundance and high rotational velocity due to mass and momentum transfer from the primary as it filled its Roche lobe as well as possibly enhanced α elements as supernova debris material.

By constructing the past flight paths of young NSs and young runaway stars, we aim on identifying the NS that was born in the Antlia SNR. For those NSs for which we consider an association with the Antlia SNR possible, we also investigate whether a different origin in a nearby young association or cluster is possible. To account for the errors on the observables as well as for the NSs unknown radial velocity, we perform Monte Carlo simulations.

With this work, we aim to extend the analysis done by McCullough et al. (2002), by investigating a larger sample of NSs and, as an additional indicator, search for a former companion candidate to the NS that was then ejected in the same supernova event that formed the Antlia SNR.

We describe our method in Section 2 before presenting the analysis and results in Section 3 and giving concluding remarks in Section 4.

2 METHOD

We use the same approach as already described in Tetzlaff et al. (2010, 2011) (see also Hoogerwerf et al. 2001). Therefore, only a brief description is given here.

To construct the trajectories of the Antlia SNR, young NSs and runaway stars (and young associations and clusters; Tetzlaff et al. 2010, 2012; Tetzlaff 2013), we apply Monte Carlo simulations by varying the observables (parallax, proper motion, radial velocity) within their error intervals. For the radial velocity of the NS, we assume a probability distribution such that the distribution of pulsar space velocities according to Hobbs et al. (2005) is satisfied. Runaway star data were taken from the Tetzlaff, Neuhäuser & Hohle (2011) catalogue for Hipparcos runaway stars (also Tetzlaff 2013). If the radial velocity of a runaway star is unknown, we vary it randomly within ±500 km s⁻¹.

In a time range between 0 (today) and 5 × 10⁶ yr (into the past, in steps of 10⁴ yr), the past separation between the centre of the Antlia SNR (adopting (l, b) = (276| $_{.}^{\circ}$|5, 19°); McCullough et al. 2002) and the NS is evaluated. Later, also runaway stars are traced back simultaneously. The smallest separation d_min for each pair of trajectories (Antlia SNR and NS or runaway star and NS) and the associated time τ in the past is stored. The distribution of separations d_min is supposed to obey a distribution of absolute differences of two 3D Gaussians (see Hoogerwerf et al. 2001; Tetzlaff et al. 2010, 2011).

The distance to the Antlia SNR is uncertain. For the Monte Carlo simulations, we adopt values of |$100^{+400}_{-40}\,{\rm pc}$| to take into account the lower and upper limits by McCullough et al. (2002) and that they prefer a distance of ≈100 pc.

The actual (observed) case is different from this simple model (no 3D Gaussian distributed positions, due to e.g. the Gaussian distributed parallax that goes into the position reciprocally, complicated radial velocity distribution, etc.). Therefore, we will adapt the theoretical formulae (equations 1 and 2 in Tetzlaff et al. 2012; here we use the symbols μ and σ for the expectation value and standard deviation, respectively) only to the first part of the d_min distribution (up to the peak plus a few more bins, see Tetzlaff et al. 2012). The derived parameter μ then gives the positional difference between the two objects. Typically, a few million trajectories are constructed throughout a Monte Carlo simulation.

This procedure was already successfully applied by Hoogerwerf et al. (2001), Bobylev (2008); Bobylev & Bajkova (2009) and us (Tetzlaff, Neuhäuser & Hohle 2009; Tetzlaff et al. 2010, 2011, 2012). An investigation of (artificial) test cases showed that it is well possible to recover place and time of the formation of an NS (Tetzlaff 2013).

3 RESULTS

3.1 Search for the related NS

Among all young (spin-down ages smaller than 50 Myr) nearby (≲3 kpc) NSs with known proper motion, 106 in total (ATNF pulsar data base;¹ Manchester et al. 2005), the projected past paths of seven NSs cross the Antlia SNR during the past 5 Myr, Fig. 1. Four of them are too distant (≈2 kpc) and were not closer than a few hundred pc to the remnant for any reasonable radial velocity. Among the other three is RX J0720.4−3125 which was probably born in the Trumpler 10 association ≈1 Myr ago (Motch, Zavlin & Haberl 2003; Kaplan, van Kerkwijk & Anderson 2007; Tetzlaff et al. 2010, 2011) but still should be considered as a candidate for an origin in the solar neighbourhood or the Antlia SNR (as it could be nearby; McCullough et al. 2002). The remaining two are PSR J0630−2834 and PSR J0953+0755. The properties of PSR J0630−2834, RX J0720.4−3125 and PSR J0953+0755 are given in Table 1.

To check whether nearby associations or clusters could host the birth places of PSR J0630−2834 and PSR J0953+0755,² we investigated close encounters with young nearby associations and clusters (Tetzlaff et al. 2010, 2012; Tetzlaff 2013). The past trajectories of both stars point to several nearby young local associations (Fernández, Figueras & Torra 2008; Torres et al. 2008); however, no convincing birth association could be found. We consider an origin in the solar neighbourhood or the Antlia SNR most likely for these stars.

The past separation between the Antlia SNR and the three NSs PSR J0630−2834, PSR J0953+0755 and RX J0720.4−3125 were then evaluated, Fig. 2.³

For PSR J0953+0755, equation 2 in Tetzlaff et al. (2012) (μ = 0) fits well the d_min distribution. Note that the obtained encounter time of ≈0.5 Myr is considerably smaller than the one claimed by McCullough et al. (2002) (τ ≈ 2-4 Myr). An encounter time of ≈2 Myr is obtained if v_r = 50 ± 50 km s⁻¹ is assumed for the pulsar (comparable to the range McCullough et al. 2002 adopted). However, then a theoretical curve with μ = 48.2 pc and σ = 25.8 pc fits well the d_min distribution rather than μ = 0. It is still possible that PSR J0953+0755 was inside the Antlia SNR, though less likely. Moreover, the resulting space velocity of the pulsar would be very small, ≈80 km s⁻¹, which is unlikely but not impossible. For the calculation using a reasonable space velocity distribution for PSR J0953+0755 (from Hobbs et al. 2005), the present NS parameters and position of the encounter are given in Table 2. As already expected from Fig. 1, the predicted encounter position of PSR J0953+0755 is only marginally consistent with the observed position of the Antlia SNR.

Although the d_min distributions in the cases PSR J0630−2834 and RX J0720.4−3125 are not well represented by equations 1 and 2 in Tetzlaff et al. 2012 (probably because the parallax error is large in both cases, 14 per cent for PSR J0630−2834, 44 per cent for RX J0720.4−3125, whereas for PSR J0953+0755 it is only 2 per cent), they suggest that both objects could have been at the same place (as the supernova) at the same time in the past. In the case of RX J0720.4−3125, the predicted encounter position is again only marginally consistent with the observed SNR centre (also seen in Fig. 1). Together with the previous result that RX J0720.4−3125 was probably born in the Trumpler 10 association, we consider a common origin of this NS with the Antlia SNR less likely.

For PSR J0630−2834, the encounter position is the most consistent with the observed coordinates of the Antlia SNR centre, making it the best candidate for the pulsar that can be associated with the Antlia SNR. The inferred ages of the pulsar and the SNR if they originated from the same supernova event are ≈0.6 Myr.

The small supernova distance of ≈ 45-90 pc implies an absolute SNR radius of ≈ 10-20 pc. For an 0.4-1.0 Myr old SNR, SNR expansion theory (Snowplough expansion, e.g. Blinnikov, Imshennik & Utrobin 1982) predicts a radius of ≈ 45-55 pc for an ISM density of n = 1 cm⁻³ and standard explosion energy of E = 10⁵¹ erg. Either the explosion energy was considerably smaller (≈10⁴⁹ erg) or the supernova occurred at a somewhat larger distance (also implying a slightly larger age). Theoretical core-collapse supernova models do not predict explosion energies <4 × 10⁵⁰ erg (Ugliano et al. 2012). For a larger supernova distance ( ≈ 100-200 pc such that the predicted size of the SNR is consistent with the observed one at an age of ≈ 1-2 Myr), the radial velocity of PSR J0630−2834 is required to be smaller ( ≈ 100-300 km s^{− 1}), inferring a spatial NS velocity of ≈ 100-300 km s^{− 1} (kick velocity).

Assuming NS radial velocities of 200 ± 100 km s⁻¹ yields a distribution d_min that is still consistent with equation 2 in Tetzlaff et al. 2012 (μ = 0, σ = 24.5 pc). The time of the encounter is ≈ 0.7-1.4 Myr in the past. The predicted distance to the supernova is ≈ 70-150 pc (as seen from Earth today). The corresponding absolute SNR radius is ≈ 15-30 pc. This is in marginal agreement with the theoretically expected value of ≈ 30-60 pc.

3.2 The former companion candidate to PSR J0630−2834

The scenario in which the pulsar PSR J0630−2834 is the compact remnant of the Antlia SNR is supported by the identification of the former companion candidate HIP 47155 for which the encounter position with PSR J0630−2834 coincides with the Antlia SNR. The radial velocity of HIP 47155 (spectral type A5/7(IIw), Houk 1982; kA3hF2mA3V, Paunzen et al. 2001) was previously unknown. Its measurement is crucial to confirm or reject HIP 47155 as former companion candidate to PSR J0630−2834. HIP 47155 was considered a runaway star by Tetzlaff et al. (2011) due to its high transverse velocity of |$31^{+6}_{-2}\,{\rm km}\,{\rm s}^{-1}$|⁠.

Spectroscopic observations of HIP 47155 were gathered using the Tillinghast Reflector Echelle Spectrograph (TRES) instrument (Fűrész 2008) on the 1.5 m Tillinghast reflector at the F. L. Whipple Observatory (Mount Hopkins, AZ, USA). Three exposures were obtained over a period of 25 d in 2013 April and May. Radial velocity standard stars were observed each night with the same setup to monitor the velocity zero-point of the instrument. The spectra cover the wavelength range ∼3900–8900 Å, and were taken at a typical resolving power of R ≈ 44 000 with the medium fibre of the instrument. The signal-to-noise ratios for the individual 3- to 5-minute exposures range between 80 and 100 per resolution element of 6.8 km s⁻¹, and refer to the region of the Mg i b triplet (∼5200 Å). All spectra were reduced and extracted using standard procedures as described by Buchhave (2010) and Buchhave et al. (2010). These templates cover a 300 Å window centred at 5200 Å, which generally contains the most information on the velocity of the star. Fig. 3 shows a portion of one of our observations.

Radial velocities were derived by cross-correlation against a synthetic template taken from a large library computed by John Laird, based on model atmospheres by R. L. Kurucz and a line list developed by Jon Morse (see Buchhave et al. 2012). The task of selecting the optimum template was somewhat complicated by the peculiar chemical composition of the star, which is considered to be a member of the λ Boo class with a spectral type of kA3hF2mA3V (Paunzen et al. 2001). To establish the best template we adopted the metallicity and surface gravity derived by Casagrande et al. (2011), which are [Fe/H] = −1.13 and log g = 3.67, and explored a range of temperatures (T_eff) and rotational broadenings (vsin i when seen in projection), which are the two parameters that affect the velocities the most. The best match to the observed spectra was found for T_eff = 8250 K and vsin i = 45 km s⁻¹. The temperature is consistent with the above λ Boo classification for the metallic lines (A3). The weighted average of the three velocity measurements is +32.42 ± 0.70 km s⁻¹, and the individual values (in the heliocentric frame) with corresponding uncertainties are listed in Table 3. The above average velocity is robust against changes in the template parameters. For example, changing the temperature within one step in our grid (250 K) leads to velocity changes that are well within the quoted uncertainties.

This radial velocity for the runaway star supports the notion that HIP 47155 is a former companion candidate to PSR J0630−2834 (Fig. 4). The predicted NS properties and time and place of the supernova event are given in Table 4. The position is close to the centre of the Antlia SNR. We therefore conclude that PSR J0630−2834 and HIP 47155 were ejected in the same supernova that formed the Antlia SNR ≈1.2 Myr ago at a distance to Earth of ≈120 pc.

4 CONCLUSIONS

We searched for the NS that was born in the same supernova event that formed the Antlia SNR. In order to account for the uncertainties in the observables as well as the unknown radial velocity of NSs, we performed Monte Carlo simulations and evaluated the outcome statistically.

The most promising candidate for an association with the Antlia SNR is the young pulsar PSR J0630−2834 (Fig. 5). The predicted kinematic age of the NS, i.e. age of the SNR is ≈1.2 Myr. This kinematic age is in reasonable agreement with the pulsar's spin-down age of 2.77 Myr (Hobbs et al. 2004). Discrepancies between the spin-down and kinematic pulsar ages of a factor of a few are well known, in particular for young and middle-aged pulsars (e.g. Pivovaroff et al. 2001; Migliazzo et al. 2002; Tetzlaff et al. 2011).

The runaway star HIP 47155 is suggested to be a former companion to PSR J0630−2834, and has been classified as a λ Bootes star (kA3hF2mA3V, Paunzen 2001). We have determined its radial velocity to be 32.42 ± 0.70 km s⁻¹. Although the majority of the λ Bootes stars are believed to be old (≈Gyr), a few per cent of them are considerably younger (a few tens of Myr; Paunzen et al. 2002), some having been found in the very young Orion OB1 association and the cluster NGC 2264 (Paunzen 2001). Thus, the young age we infer for HIP 47155 is not inconsistent with its nature and it remains a possible former companion to PSR J0630−2834.

In this scenario, the distance of the Antlia SNR is ≈135 pc. This distance is also in agreement with that the Antlia SNR lies in the region of the Vela SNR and the Gum nebula or just in front of it. The Antlia SNR adjoins the Gum nebula (Shinn et al. 2007).

Integrating the COMPTEL 1.8 MeV flux (Plüschke et al. 2001; Diehl et al. 2010) over a circle centred at |$(l,b)={(270{^{\circ}_{.}} 4^{+1{^{\circ}_{.}} 9}_{-1{^{\circ}_{.}} 9},19{^{\circ}_{.}} 2^{+0{^{\circ}_{.}} 5}_{-3{^{\circ}_{.}} 0})}$| with a radius of 12 ° and adopting a distance of |$138^{+17}_{-17}\,{\rm pc}$| yields a mass of ²⁶Al that was ejected during the supernova of ≈5.2 ± 1.6 × 10⁻⁵ M_⊙. Compared to theoretical ²⁶Al yields (from core-collapse supernovae) by Woosley & Weaver (1995) and Limongi & Chieffi (2005), this corresponds to a mass of the progenitor star of ≈ 14-32 M_⊙ (earlier than B1 on the main sequence, Schmidt-Kaler 1982; Hohle, Neuhäuser & Schutz 2010).

The former host OB association or stellar cluster of the supernova progenitor probably lies in the Vela region. The massive progenitor possibly got ejected from its birth place prior its supernova due to dynamical interactions (see e.g. Leonard 1989). It is known that about 20–30 per cent of supernova progenitors are located outside stellar groups (e.g. Mason et al. 1998; Maíz-Apellániz et al. 2004).

At a supernova distance of ≈ 110-150 pc about 1.0-1.5 Myr ago, this supernova may not be the only candidate for the supernova that produced the ⁶⁰Fe found on Earth (Knie et al. 2004; Fitoussi et al. 2008), as it might be too distant and/or too recent. However, this supernova may have produced some of the ⁶⁰Fe found and contributed to reheating the Local Bubble.

We are grateful to P. Berlind and G. Esquerdo for help with the spectroscopic observations of HIP 47155. We thank R. Diehl for kindly providing us the COMPTEL 1.8 MeV map and fruitful discussions. We would also like to thank the anonymous referee for useful comments.

NT acknowledges Carl-Zeiss-Stiftung for a scholarship. GT acknowledges partial support for this work from NSF grant AST-1007992. NT, RN and MMH acknowledge support from DFG in the SFB/TR-7 Gravitational Wave Astronomy. This work has made use of the Australia Telescope National Facility (ATNF) pulsar data base and the SIMBAD, operated at the Centre de Données astronomiques de Strasbourg (CDS).

REFERENCES