4U1700-37

4 Evolutionary history of 4U1700-37

The stellar parameters for both primary and compact companion determined via non-LTE modeling and Monte-Carlo simulation raise many important questions regarding the evolution of single and binary massive O stars and their ultimate post-SN fate. However, such questions are complicated by the uncertain evolution of massive stars after leaving the main sequence and the role that binarity and associated - possibly non-conservative - mass transfer plays in modifying this; for instance the time at which the hydrogen rich outer layers are lost exposing the helium core plays a critical role in determining the final pre-SN mass of the star (e.g. Brown et al. 2001 and references therein).

Heap & Corcoran (1992) propose an initial 80  $M_{\odot}+$ 40 $M_{\odot }$ binary system with subsequent evolution via case B mass transfer after $2.6\times10^6$ yrs, proceeding for 10⁴ yrs (via Roche-lobe overflow; RLOF). After the mass transfer the initially more massive star has lost enough material due to the combination of wind driven mass loss and the brief period of non-conservative RLOF to become a WR star, which subsequently explodes as a SN.

Based on their identification of the Sco OB1 association as the birthplace of HD 153919, Ankay et al. (2001) propose a lower initial mass of the SN progenitor ($\geq$30 ⁺³⁰_-10 $M_{\odot }$) based on the turnoff mass for the proposed $6\pm 2$ Myr age of Sco OB1 at the time of the supernova. Assuming conservative Case B mass transfer they derive an initial mass of at least 25 $M_{\odot }$ and suggest the short orbital period Wolf-Rayet (WR) binary CQ Cep/HD 214419 as a possible example of the progenitor system. However they note that the assumption of conservative mass loss might be incorrect and highlight the non-conservative scenario of Wellstein & Langer (1999). Such a scenario is attractive since the loss of substantial quantities of mass and angular momentum naturally lead to short period binaries (assuming both components do not merge). However, common envelope evolution is poorly understood and therefore somewhat limits our ability to quantitatively reconstruct the pre-SN evolution of the binary.

Despite these uncertainties we can address the general evolution of the binary in some detail. Our present mass estimate for HD 153919 of $M_{\ast} = 58^{+11}_{-11}$ $M_{\odot }$ suggests a mass for the SN progenitor of the order of $\ga$60 $M_{\odot }$, at the upper range of that proposed by Ankay et al. (2001). The short orbital period of HD 153919/4U1700-37 favours non conservative evolution probably via case B mass transfer. While mass loss via the stellar wind of the SN-precursor during the WR phase will lead to a widening of the orbit, a favourable SN kick may overcome these problems.

Figure 5: Histogram of the results of the Monte Carlo simulations for the mass of the O6.5Iaf+ primary HD 153919 - the results indicate a mass in the range of $58\pm 11$ $R_{\odot }$ consistent with evolutionary predictions and the mass estimated from our determination of log g=3.45-3.55 (Sect. 2.2).

 

 

Table 2: Physical parameters of 4U 1700-37. Those with a $\pm$ have a Gaussian error distribution while those without are assumed to have a uniform distribution.

Parameter	Value
$R_{\ast}$	21.4-23.2 $R_{\odot }$
$\Gamma$	0.5-1.0a
$\Omega$	0.8-1.0a
e	0.0b
$\theta_{\rm E}$	$28.6 \pm 2.1$ degreesa
P	$3.411581 \pm 2.7 \times 10^{-5}$ daysa
$K_{\ast}$	$20.6 \pm 1.0$ km s-1b

^a Rubin et al. (1996); ^b Hammerschlag-Hensberge et al. (in prep.).

Figure 6: The location of HD 153919 in the Hertzsprung-Russell diagram. The evolutionary tracks of Lejeune & Schaerer (2000) for starsof a given initial mass are indicated (up to the phase of core-helium burning). The numbers along the tracks show the decrease in $M_{\ast }$ with time due to wind losses. The plus signs indicate the end of core-hydrogen burning. The diagonal dashed lines are lines of constant radius.

The alternative Case C evolution appears unlikely given that the SN will occur several thousand years after the end of mass transfer/loss. This time period appears unrealistically short given that this will not allow sufficient time for the helium mantle to be removed to expose the C/O core (i.e. the star will not pass though a WC phase). Therefore the SN progenitor would have a large mass at the point of SN; for a SN progenitor with an intial mass of 60 $M_{\odot }$we might expect the mass to be of the order of 30 $M_{\odot }$ (the maxium helium core mass). This implies the loss of a very large quantity of material in the SN, contradicting the estimate of Ankay et al. (2001) that only $\sim$9 $M_{\odot }$ was lost in the SN (and we might also expect such a scenario to lead to a very high mass compact object, cf. Brown et al. 2001).

The high mass implied for the SN precursor suggests that such a star would be likely to evolve through a Luminous Blue Variable (LBV) phase rather than a Red Supergiant (RSG) phase on its way to becoming a WR star (stars with initial masses $\ga$40 $M_{\odot }$ likely avoid the RSG phase). Such an evolutionary path is likely to prevent mass transfer onto HD 153919 via RLOF (Wellstein & Langer 1999). Significant accretion of material by HD 153919 via RLOF seems implausible in any case, as this would lead to a large orbital separation and period (Wellstein & Langer 1999). Instead, the formation and subsequent ejection of a common envelope (despite the SN precursor avoiding the RSG phase) and simultaneous reduction in orbital period and binary separation is suggested. Such a scenario therefore implies that the present day mass of HD 153919 forms a lower limit to the mass of the SN precursor, subject to the possible accretion of a small quantity of material directly from the wind of the SN precursor (see below).

After the ejection of the outer layers of the SN precursor we are left with a short orbital period WR+O star binary. Support for this scenario is provided by the possible overabundance of carbon (or rather the lack of significant C depletion as might be expected for CNO processed material; Sect. 2.2) in HD 153919. The carbon rich material must have originated in the SN precursor during a carbon rich WC stage, independently suggesting a rather high initial mass for the SN precursor. Subsequent mass transfer would then have to occur via direct wind fed accretion, with the wind of the WC star impacting directly on the surface of the O star. Despite the high mass loss rate of the O star ( $\dot{M}=9.5\times 10^{-6}$ $M_{\odot }$ yr^-1; Table 2) the possibility of such accretion is suggested by hydrodynamical simulations of colliding wind binaries (Gayley et al. 1997; Dessart, Petrovic & Langer, in prep.).

We may exclude the overabundance in nitrogen originating via direct wind accretion during the WN phase of the SN precursor. For a nitrogen overabundance in HD 153919 of $\sim$9, the nitrogen mass fraction, $X_{\rm N}=0.01$, is the same value as is found in the winds of WN stars which implies that HD 153919 would have to accrete 18 $M_{\odot }$ from the SN precursor during the WN phase to produce such an overabundance. Given that this is unreasonably high, we suggest that the excess nitrogen probably originated from internal, rotational mixing (nitrogen overabundances are not unusual for O stars).

Several short period WR+O star binaries are known and could provide analogues to the precursor of the present binary configuration. At present CQ Cep does not fit particularly well ( $M_{\rm WN}=21$ $M_{\odot }$, $M_{\rm O9}=26$ $M_{\odot }$, P=1.6 days) - in a few 10⁵ yrs when the WN star has lost mass and has evolved into a lower mass WC star (and the period has lengthened) it may provide a better model for the system. However, of the known WC+O binaries, HD 63099 ( $M_{\rm WC}=9~M_{\odot}$, $M_{\rm O7}=32~M_{\odot}$, P=14 days) could evolve into a HD 153919/4U1700-37 like binary if the SN kick is favourable. Other known WR+O star binaries that could form similar systems are HD 152270 ( $M_{\rm WC}=11~M_{\odot}$, $M_{\rm O5-8}=29~M_{\odot}$, P=8.9 days) and HD 97152 ( $M_{\rm WC}=14$ $M_{\odot }$, $M_{\rm O7}=23~M_{\odot}$, P=7.9 days); in all three cases the present mass of the WC star is $\ga$9 $M_{\odot }$ as suggested for the SN precursor by Ankay et al. (2001) on the basis of the present mass of the compact companion and the current space velocity of the system. Therefore, of the six WC+O binaries with known masses (van der Hucht 2001) three are found to have parameters consistent with the presumed pre-SN stage of 4U 1700-37.

The results of the Monte-Carlo simulations suggest that the SN formed a compact object (see Sect. 5) with a mass in the range $2.44\pm 0.27~M_{\odot}$. As massive binary models show that Case B primary stars of larger initial mass evolve to a larger final mass, and that Case A primaries end up less massive than Case B's (cf. Wellstein & Langer 1999; Wellstein et al. 2001) we can conclude that the remnants of all Case A/B primaries initially less massive than about 60 $M_{\odot }$ are less massive than about 2.5 $M_{\odot }$.

Theoretical predictions suggest that a 60 $M_{\odot }$ star in a close binary system is capable of producing either a low or a high mass compact object depending sensitively on the wind mass loss rate adopted for such a star during its WR phase; a variation of only a factor of three in the WR mass loss rate leading to compact object masses in between 1.2 and 10 $M_{\odot }$ (Fryer et al. 2002). This result shows that given the present uncertainties in WR mass loss rates, the relatively low mass found for the compact star in 4U1700-37 is not in conflict with the evolutionary scenario proposed above (and also argues for a relatively high WR mass-loss rate).

Therefore, given the low mass of the compact companion in 4U1700-37, it seems to be difficult to explain any of the (high mass) galactic black hole binaries as being produced through the Case A/B channel (cf., Portegies Zwart et al. 1997) except for those with the most massive SN-progenitors (Brown et al. 2001). Indeed, evolutionary scenarios invoking Case C mass transfer (Brown et al. 1999) seem to be required to explain the high-mass black holes in low-mass X-ray binaries.

4U1700-37